Andrew,
Am I right to think that you have solved this now?
If not, you need first to turn File Sharing on in:
System Settings (or System Preferences if you’re running a version of macOS before 13 (Ventura) ) > General > Sharing > File Sharing
You will specify the single folder (on macOS) into which you want Roon to save its backups.
This is an operation specific to your MacBook and applies for almost any time you need to allow access to files on your host machine by other systems.
Let’s start there; have you successfully done that, Andrew? I suspect you have.
Although you should avoid spaces in (folders’) filenames… RoonBackup, not Roon Backup.
After that, you will want to tell Roon (as in the Roon dialogue ‘Choose folder Add network share’ of which you provided those two screenshots) the location which Roon can now see and use to back its database up to. (Just to be sure: this is not your collection of music, but Roon’s internal database that manages it.)
Although on macOS this location is denoted by a URI like this:
smb:**//**192.168.nnn.nnn/RoonBackups
At this stage in Roon, the slashes must be backslashes, like this:
smb:**\**192.168.nnn.nnn/RoonBackups
And, Yes, the examples in that post are specific to that network. You will use your MacBook Air’s own IP address. Please say if you need help in determining this.
Since you’ve had some success with this (and, I hope, taken note of what @Rugby posts), please let us know what stage you’re at with the above steps; and we’ll take it from there. Good luck!
BTW I strongly suggest steering clear of DropBox.