Can't add Guest Access Portal from cnMaestro to my Master Controller AP

Could I please ask how you have set the devices up?

Has it been done through AutoPilot? See here as an example:

Or has it been configured as a standalone device? See here as an example:

I'm just trying to work out how your network is currently configured then I can tell you how to onboard it to cnMestro to use the cnMaestro guest portal.