I installed High Sierra on my Mac Mini (SSD), no bother.

As my partner was still running El Capitan on her MacBook Pro (old, but upgraded to a 1TB SSD a year or two back), I thought I'd upgrade her machine too, just so we were on the same page.

When I got to the install button, the machine told me it couldn't install on 'this type of drive'.

Any ideas why, or any ideas as to a work-round?