Hello ford103,
Thank you for reaching out to Apple Support Communities.
These steps can help you set up single app mode:
"Start Single App Mode in Apple Configurator
Single App Mode forces the selected app to open on the supervised device and prevents the use of other apps. When this payload is active, the app reopens immediately after the device is restarted.
- In Apple Configurator , select one or more devices or Blueprints on which you want to enable Single App Mode.
- Do one of the following:
- Choose Actions > Advanced > Start Single App Mode.
- Control-click the selected devices or Blueprints, and choose Advanced > Start Single App Mode.
3.Select which app will be the only app available for use. Click Options to view the features available in Single App Mode.
4.Choose any options, click Apply, then click Select App."
More details can be found here: Start Single App Mode in Apple Configurator
If the issue persist, can you provide a few more details? What is your exact macOS version? What happens when you attempt to set up Single App mode using Apple Configurator?
Cheers!