Can somebody explain what 'Overallocation' means in context of file system? I constantly see this message when trying to resize APFS partition, disk checks don't find any problems.
I would have commented on Peku2455's answer, but I don't have 50 rep. So here is the solution, based on Peku's answer (which is missing the -o switch to fix overallocation)
In Terminal in Recovery Mode (press Command+R while booting, select Utilities -> Terminal):
diskutil unmountDisk /dev/disk0 fsck_apfs -oy /dev/disk0