Ubuntu Touch for surya/karna is based on Halium 10 and Android 10 Ubuntu Touch rootfs so it requires an Android 10 base firmware before it can be flashed on your phone. While custom ROMs like LineageOS 17.1 might work, they are untested and not recommended. Instead, use one of the MIUI Android 10 based firmware from below.
Pay attention to your device name before downloading the firmware below. Make sure you download the right firmware for your device, the correct region, the firmware type is fastboot, and the Android version is 10.0!
Poco X3 NFC (surya): LINK
Note: For Poco X3 (karna) use the link above and download firmware for POCO X3 India