Permissions Issue on Sonoma

I hope that someone can point me in the right direction. 

I'm using LR Classic on an M3 MacBook Pro. Both LR and the OS are up to date.

My images are contianed on an external hard drive and my catalogue backs up to Dropox. 

For no readily apparent reason I am now getting an error message as follows:-

I have tried to change permissions in the OS, but it says I don't have the permissions to make such changes. 

I've given LR  Full Disk permission but to no effect. I get the same error message. 

This problem has come out of the blue and so I'm very confused.

I would be grateful for some guidance on resolving this issue.

Thanks

Stephen

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