Keyboard

What keyboard is it? If it's an Apple wireless keyboard that is rechargeable connect it with the charging USB cable so it will run like a wired keyboard. Then Reset NVRAM or PRAM on your Mac - Apple Support 2 - 3 times in succession.

If that doesn't help with the keyboard still wired via USB boot into Safe Mode by booting with the Shift key held down and check there to see if the problem persists.  Reboot normally and test again.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

CDB917 wrote:

Letters and characters not actioning correctly.

Can you provide some specifics about the incorrect characters? What do you get when you type asdfg?

Try another keyboard.