I wish to delete several messages but using Shift or command keys does not allow selection more than one.
Pressing shift key doesn't not allow me to select multiple messages to delete. It works on my phone but not my iBook.
MacBook, macOS 10.13
Start with this...
Safe Boot, (holding Shift key down at startup), does the problem occur in Safe Mode? Could take 10 minutes or more.
Safe mode attempts to repair Disks & clears lots of caches & loads safe Drivers, & prevents loading of 3rd party extensions, so if Safe Mode works try again in regular boot.