tmutil listlocalsnapshotdates / |grep 20|while read f; do tmutil deletelocalsnapshots $f; done
Wait until it's done deleting the local snapshots. Restart your computer and it will create a new local snapshot.
Alternatively:
tmutil listlocalsnapshots /
and with
tmutil thinlocalsnapshots / 9999999999 1
you can delete all local snapshots and release the storage space.