Whenever I try to create a base flange from a tab. I get a message that says "The highlighted entity contains points that are coincident but not merged. What does this mean? How can I fix it.
What I do in that case is select a point (ie, in a corner) and see if I can drag it. If so, I merge it to my other point. Sounds like with the sheetmetal this may be converted entities where you selected an edge at a time (in which case the points on the ends of the converted edges would be coincident without being merged).
Test by dragging points. You'll know which sequence you converted the entities, so this should give you a clue as to where to start.
To avoid this in the future, select all your edges at one time (or as many as you can practically do) and then convert them to line segments.