Reset and reformat external SSD to APFS
Open Terminal and get the details

diskutil list


Below I assume the external "Apple_Boot/Recovery HD" has the disk identifier disk3s3 and the APFS container disk3s2.
Remove "Recovery HD".

diskutil eraseVolume "Free Space" %noformat% /dev/disk3s3

Expand "Container disk4" to fill the free space.

diskutil apfs resizeContainer disk3s2 0

Delete volume (here the volume "Senza titolo")

diskutil apfs deleteVolume disk4s2