Can't build .tmx files with Monogame.Extended Content Pipeline

Hey, newbie here.

I’m trying to build a .tmx file with Monogame.Extended.Content.Pipeline
Here are all the errors and info:

As you can see, I can’t even build tiles.png, but that’s “fine”, since I can with the normal Pipeline. I don’t know if the extended pipeline should be able to build “normal” files.

I was able to develop a small game with Monogame so this is something related to the extended content pipeline.

MonoGame.Extended 3.8.0
MonoGame.Extended.Content.Pipeline 3.8.0
MonoGame.Extended.Tiled 3.8.0
Monogame Content Pipeline
msbuild 16.6.0
dotnet 3.1.409
Ubuntu LTS 20.4