This has happened to me more than once. Somehow I get a sketch into a state that it no longer allows constraints. For example, in sketch 3 of the attached file if I draw a circle and a line anywhere in the sketch and try to constrain them with a tangent or midpoint constraint I get an error. I can open a new sketch in the same project, draw a circle and line and then constrain them with no problem. The last time this happened the only way I could find to resolve the issue was to scrap the sketch and start over. I am using Proefssional V26.