50GB of disk space missing
You previously installed Windows (or another OS) and deleted it afterwards - except an orphaned Apple_BOOT or 2nd EFI partition (disk0s3 - 134.2 MB).

The 50 GB are simply unallocated disk space.

To expand your APFS container you have to remove the third partition and resize the container:

First convert the third partition to free space:

sudo diskutil eraseVolume "Free Space" %noformat% /dev/disk0s3


Then expand your APFS container to the max (0 is a magic size here):

diskutil apfs resizeContainer disk0s2 0