OpenTK error

#### Issue details

I'm getting the issue 'The type or namespace name 'OpenTK' could not be found (are you missing a using directive or an assembly reference?)' right outside the box.

I followed the instructions and have installed all the required packages. what do i need to do to fix this?
I haven't made any changes to the code yet.I am unable to fix this or build it.

Start with the reported build error and verify the required package or project reference for OpenTK. Done means Mission Planner builds without the missing namespace error; the issue names no file, test, or entry point to target.

Written by the indexing model from the issue text.