iPhone 8 backup error: iTunes could not connect to this phone

I do not understand the error message. His laptop is a 2013 Mac Air running High Sierra 10.13.6

I have never backed up his phone, yet don’t know if I’ve backed up my own iPhone8. Where to find my backup status? How to create a back up for his iPhone once I’ve understood ‘Could not allocate a resource’? Is the allocation of a resource due to the fact that his phone has never been backed up on this laptop? Obviously I’m a novice at understanding Apple-Speak!

[Re-Titled by Moderator]