reported on forum and I can repro: https://community.freepbx.org/t/unable-to-add-a-system-recording-see-the-console-for-more-details/82628/4
Can not repro in FreePBX 15, appears to be a 16 only issue
When attempting to add/edit a system recording with the 'ulaw' option toggled for generating the recording, the submit fails with:
Browser console shows this error:
Unable to find an intermediary converter for /var/lib/asterisk/sounds/en/minions-not-answering-leave-message.ulaw [/var/lib/asterisk/sounds/en/minions-not-answering-leave-message.ulaw.ulaw] (anonymous) @ recordings.js?load_version=16.0.6:139
work around is to not generate recording in ulaw format until this is resolved.