I suspect that something went wrong during the installation of the server. Please try to install it again. In the Squeezebox Server plugin you can click on „show available versions“ and then start the installation.
If it still doesn’t work, there is probably something wrong with the image. In this case, please burn the current Buster image onto the SD card, start the pi, activate your licence and then install the Squeezebox server again.
We do not currently offer a separate display image with Buster. So you would have to install the RPI display plugin yourself when using the blank Buster image.