Enlarging APFS OS Volume - Unknown partition scheme
You need to boot to macOS Recovery. Instructions for booting to macOS Recovery over the internet can be found here. Once booted to macOS Recovery, open a Terminal application window and enter the following commands.

diskutil unmountdisk disk0
gpt remove -i 2 /dev/disk0
diskutil unmountdisk disk0
gpt add -i 2 -b 409640 -s 101021416 -t 7C3457EF-0000-11AA-AA11-00306543ECAC /dev/disk0


After booting back to macOS, the commands given below will add the space formally used by linux to the partition where macOS is installed.

diskutil eraseVolume free none disk0s3
diskutil apfs resizecontainer disk0s2 0


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