YT Subs to WL

app-script.js

function findPlaylist(compare) {
  let pageToken;
  while(true) {
    const list = YouTube.Playlists.list(['snippet'], {
      "maxResults": 50,
      "mine": true,
      ...(pageToken && { pageToken })
    });
    
    const item = list.items.find((row) => compare && compare(row));
    if (item) {
      return item;
    }

    if (!list.nextPageToken) {
      return null;
    }

    pageToken = list.nextPageToken;
  }
}

function addPlaylist() {
  const title = `wl__${(new Date().getMonth()) + 1}`;

  const item = findPlaylist(({ snippet }) => snippet.title === title);

  if (item) {
    return item;
  }

  return YouTube.Playlists.insert(
    {
      'snippet': {
        title,
      },
      'status': {
        'privacyStatus': 'private'
      }
    }, 
    'snippet,status'
  );
}

function insertVideo(playlistId, videoId) {
  return YouTube.PlaylistItems.insert(
    {
      "snippet": {
        playlistId,
        "resourceId": {
          "kind": "youtube#video",
          videoId
        }
      }
    },
    'snippet'
  );
}

function run() {
  const spreadsheet = SpreadsheetApp.getActiveSpreadsheet();
  const sheet = spreadsheet.getSheets()[0];

  const lastRow = sheet.getLastRow();

  console.log({ lastRow });

  if (lastRow < 1) {
    return;
  }

  // 199 because 1 request for insterting playlist
  const lastIndex = lastRow > 199 ? 199 : lastRow;

  console.log(`get range: A1:A${lastIndex}`);

  spreadsheet.toast(`get range: A1:A${lastIndex}`, "fill playlist");

  let range = sheet.getRange(`A1:A${lastIndex}`);

  const rows = range.getValues().flat().filter(Boolean);

  console.log({ willProcessing: rows.length });

  if (rows.length) {
    let lastAdded = 0;
    let error;
    let lastVideo;
    try {
      const pl = addPlaylist();
      rows.forEach((videoId, index, arr) => {
        if (!videoId) {
          return;
        }

        try {
          insertVideo(pl.id, videoId, arr.length - index);
          lastVideo = videoId;
        }
        catch(err) {
          if (!err.message.includes('Video not found')) {
            throw err;
          }
          
          console.log('video not found', videoId);
          lastVideo = videoId;
        }
        lastAdded = index + 1;
      });
    } catch(err) {
      error = err;
    }

    console.log({ lastAdded, lastVideo });

    if (lastAdded > 0) { 
      if (!lastVideo) {
        lastVideo = sheet.getRange('B1:B1').getValue();
      }

      const row = sheet.getLastRow() || 1;

      if (lastAdded >= row) {
        lastAdded = row - 1;
      }

      console.log({ row, lastAdded });

      if (lastAdded > 0) {
        sheet.deleteRows(1, lastAdded);
      } else {
        sheet.getRange('A1:A1').setValue('');  
      }

      sheet.getRange('B1:B1').setValue(lastVideo);
    }

    if (error) {
      throw error;
    }
  }
}

function init() {
  const spreadsheet = SpreadsheetApp.getActiveSpreadsheet();
  spreadsheet.removeMenu('YouTube');
  spreadsheet.addMenu('YouTube', [
    {
      name: 'fill playlist',
      functionName: 'run',
    }
  ]);
}

function onOpen() {
  init();
}

snippet.js

const targetRootId = 'contents';

const lastVideoSelector = 'ytd-rich-grid-row:last-of-type ytd-rich-item-renderer:last-child';
const allVideosSelector = `.ytd-rich-grid-renderer#${targetRootId} ytd-rich-grid-media`;
const rootNodeSelector = `ytd-rich-grid-renderer > #${targetRootId}`;

const delay = (ms = 100) => new Promise((resolve) => {
    setTimeout(() => {
        resolve(true);
    }, ms)
});


(async function() {
    let neededVideoId = prompt('Enter id of the last video', '');

    if (!neededVideoId) {
      alert('The last vidoe id can not be empty');
      return;
    }

    const checkWatched = async () => {
        await delay(3e3);

        console.log('find video by id:', neededVideoId);
        const found = document.querySelector(`a[href^="/watch?v=${neededVideoId}"]`);

        if (found) {
            console.log('found, stop observer');
            observer.disconnect();
            stageSecond();
            return;
        }

        const lastVideo = document.querySelector(lastVideoSelector);
        console.log('last video:', lastVideo);
        if (lastVideo) {
          lastVideo.scrollIntoView();
        }
    };

    let timer;
    const runWaiter = () => {
        if (timer) {
            clearTimeout(timer);
        }
        timer = setTimeout(checkWatched, 2e3);
    };

    const stageSecond = async () => {
        let videos = Array.from(document.querySelectorAll(allVideosSelector));

        if (neededVideoId) {
            const index = videos.findIndex(item => item.data.videoId === neededVideoId);
            videos.splice(index, videos.length);
        }
        
        videos = videos
          .filter(item => !item.data.isWatched)
          .reverse()
          .map(video => `<tr><td>${video.data.videoId}</td></tr>`)
          ;

        document.body.innerHTML = `
          <pre style="background: white; color: black">
            ${videos.join('\n')}
          </pre>
        `;
    };

    const itemsContainer = document.querySelector(rootNodeSelector);

    const mutationListener = (mutationList, observer) => {
        const list = mutationList.filter(item => item.target.id === targetRootId);

        if (list.length > 0) {
            runWaiter();
        }
    };

    const observer = new MutationObserver(mutationListener);

    observer.observe(itemsContainer, { childList: true, subtree: true });

    await checkWatched();
})();

To see that happened in the script need to visit to execution and open an execution with an error to see a reason.

I don't know what is the reason for that error, you might add console.log to the script and try to debug by yourself.