I am trying to import and then edit a design in Tinkercad, however, the program will not let me ungroup the model and make changes to it.

Does anyone know how this problem can be resolved?

Many thanks!

Unfortunately if the design wasn’t originally grouped in the same TinkerCAD file, it doesn’t know the original shapes in order to separate them. I would recommend trying MeshMixer, and its “separate shells” feature.