What seems to be the problem?

Why don't wait until 27.1 ported to flatpak? Flatpak is recommended by developer if you're using Debian (me LMDE). They should've maintained this repo. 

My suggestion at this point is to uninstall 0 A.D. by using the command line (terminal). The reason for that is to purge 0 A.D. from your system. Uninstalling 0 A.D. through flatpak may not delete everything. As a quickie suggestion ask both Grok and ChatGPT the following: "How to uninstall 0 A.D. by using the command line with Linux Mint.". You can also make a post on the Mint forum website. There are different methods for installing 0 A.D. your problem may be associated with that. Grok provided the text below: "grep" searches for the presence of 0 A.D. in each of the installation methods. Theoretically you should only see one listing. If there are more listings, it is possible that they are conflicting. It will be necessary to uninstall (purge) 0 A.D. from your computer and then choose one method to install. Both Grok and ChatGPT provide advice on that. I think the ChatGPT gave better advice. One piece of advice that ChatGPT listed as optional was:"sudo apt purge 0ad 0ad-data". I would not consider that as optional. But you need to be aware that it removes your user data. Since you are just getting the program setup, the user data, at this time, should not be an issue. Of course I need to add the disclaimer that any advice from either Grok or ChatGPT can be wrong (that includes me too), that you need to be careful, and you should think through any command before executing.

I am on Linux Mint XFCE, and I have installed the flatpack version from the software manager but cannot use latest alpha. Although update manager is noticing me the game has a new update, it seems there is something wrong as it does not update the game when download finishes. Also I am a new Linux user, just installed the OS some days ago, so I might be missing something. Anyway I am going to use 27.0 until a find the way to build the 27.1 as @ffm2 suggested to me.

Thank you so much!

Oooh that is the former, outdated git clone of our historical SVN repo. It is archived since summer 2024. The current repository is at https://gitea.wildfiregames.com/0ad/0ad

What version of Linux is @Essential Strategyusing? With Mint, you can download 0 A.D. through flathub (flatpak) from the software manager. See image below. If not using Mint, check your distribution for the ability to access flathub.