Hello Pedro,
Hope you're doing well.
May I ask you which MABS version are you using?
In order to build your apps with plugins using AndroidX library, you will need to use MABS version 6.3 or later.
With that said, AndroidX is not enabled by default in MABS version 6.3, so you'll need to enable it in your app's Extensibility Configurations:

For MABS version 7.0 Beta, AndroidX is enabled by default, so you shouldn't have this issue.
I just did a quick test for this plugin using MABS version 6.3 and AndroidX enabled and I managed to generate the APK successfully:

You can check this documentation that may help you.
Kind regards,
Rui Barradas