A common error that occurs after years of use is that one or more keys no longer respond, although everything mechanically seems to be in order. As a workaround, you can replace individual keys by pressing them with a Ctrl+x combination. For a long time after the BACKSPACE key failed, I continued to work with the key combination CTRL+H.
Unfortunately, this error cannot be repaired with justifiable effort, a replacement keyboard is due.
It is possible, with a little finesse, to switch keycaps if only one keyboard with a different language layout is available.