Hello. I'm new to MonoGames and this errors started to pop up

[jnoyola]

Same recommendation as this post

@jnoyola

If it can’t find the namespace, that usually means it couldn’t restore NuGet packages. Try right-clicking on the solution node in the solution explorer and force it to restore. Also check the Output tab and select Package Manager from the dropdown to see if there are any errors.

Also when was the last time you updated VS 2019? That little notification bell in the bottom right corner could mean you have some updates, which may mean you don’t have the .NET Core 3.1 SDK installed.