In group options you can tick the "allow redundant constraints" checkbox to suppress the error. However, SolveSpace's constraint solver does not handle many kinds of redundant constraints very well, which is why this is an error by default. Because it may significantly impair convergence, I recommend avoiding this option other than for assemblies.

@dims12 You can also select one of the angle constraints and convert it to a reference dimension "REF" on the toolbar. Then it will show the angle but not add the redundant constrain. Version 3 will do this automatically when the 3rd constraint is added.