Dear Francesco Milani,
Thank you for posting in the Microsoft community.
I understand that you are unable to add plugins on your Android phone. I'm happy to offer some explanations.
Firstly, I'd like to know whether the personal mailbox you are using is a Microsoft domain mailbox (Outlook/Hotmail/Live/Msn) or a third-party mailbox (Gmail / Yahoo! / etc.)?
As far as I know, this problem may occur on Android phones. There may be regional restrictions when directly adding plugins in the Android Outlook app, but this won't happen on the Mac system.
I suggest that you use the Google Play Store with the location set to the United States to download the Outlook app and try adding plugins again to see if this prompt appears.
Please believe that I'm really eager to solve your problem, so I hope to get your cooperation.
Thank you for sharing more detailed information with me. I'll keep an eye on this thread and give a further reply as soon as possible after receiving your reply. I'm looking forward to your reply and wish you all the best.
Best Regards,
Ulrica.W - MSFT | Microsoft Community Support Specialist