I have a client record but it doesn't allow me to access it as if it were deleted, yet it's linked to other transactions, and this message appears when I hover the mouse over that client's name. Does anyone know what it could be?

are you able to click to it or access the record directly?

If you can click on it but it doesn't access the record, it just sends me an error message. That client has related transactions, so it seems strange to me that it doesn't exist.

you should to be able to find out the internal ID of that record from the link. Go to your instance and open up another record of similar type --> replace the internal id (numbers) in the URL of the similar record and hit enter. If the record in question exists then it should load.