When attempting to suppress a feature that is being referenced by another features sketch via a relation, the following error is shown:
You attempted to change a field that is currently locked by a design table.... Blah blah blah
The second feature that has the sketch that relates to the first feature is design table controlled. Whether the design table is locked or not, it does not matter, you get the same error. The solution is to find the sketch with the relation that is referencing the feature and sketch you are trying to suppress. The way to do this that I can think of is look at the parent child window to see what its relationships are. It would be nice however to see these relations that are referring to it in the relations window in the sketch. You can check the sketches external relations but not relations from other sketches that are referring to it. Maybe this could be added in a future release. Correct me if I am wrong but I see no other way to find external relations that are referring to your current sketch but by the parent child window. Now in an assembly, mates are seen no mater what part you choose, you see any mates referring to those parts, it would be nice if sketch relations operated this way too.