Welcome to Android Central! Just to be clear -- you're talking about Samsung's Find My Mobile, and not Google's Find My Device, correct? Google's doesn't have an option to remotely unlock -- only to remotely lock.
Hmm. I'm not too familiar with Samsung's Find My Mobile. Are the 2 devices used on a work account? If so, the security policy might prevent remote unlock.
See my screenshot, if the first option isn't on, than that might be the culprit.
Other than that, I don't see why your devices don't have that option.
Unless they're older or unsupported devices. In my case, I have an old Samsung Tab 3 that doesn't even show up.