Does your name happen to have a space inside it? If so, that’s the issue. In the current released MonoGame version, there’s a bug where you can’t have a space in your path.
That bug has been fixed in the develop branch.
You can manually fix it in your local files.
Find your local nuget packages: ~/.nuget/packages more info: https://docs.microsoft.com/en-us/nuget/consume-packages/managing-the-global-packages-and-cache-folders
The task package should be here: ~/.nuget/packages/monogame.content.builder.task/3.8.0.1641/build
Apply this fix: https://github.com/MonoGame/MonoGame/commit/886b639294a357e11e8afd601c4fe1dac9ad6ef2.