iMac won't connect to MacBook Pro

My iMac and MacBook Pro are side by side, they both show in each finder but I can't connect them even when using the correct password with Registered User ticked. This has been happening for quite a while now, the iMac runs High Sierra, the MBP runs Mojave but I don't think that is the problem. I suspect that it';s something to do with the MBP from when I Migrated when setting up from a previous Mac. Is there a method of checks to see if the cause can be pinned down?

Thanks