Hi apple_music_maker,
It sounds like GarageBand isn't picking up your digital piano or keyboard. We're here for you, and we want to help any way possible.
To help us narrow down what may be causing this issue, can you let us know if you've been able to use this instrument with GarageBand successfully in the past? If you have been able to use it in the past, when did this issue first begin?
Which macOS version is your Mac running? This page explains how to find the exact macOS version number: Find out which macOS your Mac is using
Did you connect your instrument using these steps? GarageBand for Mac: Connect a music keyboard
Also, in case you haven't seen it yet, the following page has some great information and tips for GarageBand: GarageBand Help
Thanks!