Cannot resize APFS container after updating to Big Sur

Finally found a fix! Deleting snapshots using tmutil didn't work but I deleted them with diskutil from Recovery: diskutil apfs listSnapshots <Macintosh HD's disk id> and then diskutil apfs deleteSnapshot <disk id> -uuid <snapshot uuid from the previous command> for every snapshot. I had 3, two of them were deleted and one returned "insufficient privileges" but it doesn't matter, now everything works. Do it at your own risk, I don't know whether it will work on your Mac.

sudo diskutil apfs resizeContainer disk0 0

You could vary the statement if you have a specific size in mind but you also just set it to 0 and it will grow to maximum physical disk size.

You can do this after copying the Time Machine image to a new disk for example.