Hello Richen
From my experience, I would recommend you to generate APKs locally in your machine.
The AppCenter free tier is not robust enough to generate APKs/AABs or IPAs from Mendix projects. I would only use AppCenter in the case of a payed account (with more resources in terms of CPU and available build time) and to generate signed IPAs for IOS.
If you have Windows and your target app is Android, you can generate your APK/AAB localy faster than generate on the cloud with AppCenter. See the procedure below:
If you have a Mac, it is also possible generate IPA locally, with little extra effort to copy the native project to your Mac beforehand.
But my prefered way to generate APKs/AABs is via command line, where I have (almost) full control of the steps.
https://vstram.medium.com/mendix-b554e0678c72