Every time I import grabcad designs into spaceclaim I get a license invalid error.
Have you tried contacting Spaceclaim direct ? Their support is quite efficient.
Can you send me the link to one of the models you are trying to import so I can check on my side ?
Do you have the full version of SpaceClaim? Or do you just have the free version DesignSpark Mechanical?
You can open Step files in DSM but they are locked. To unlock them draw a square or circle anywhere on the drawing, Pull to make it a solid. In the structure tree right click the top component and un-pick the 'lock tickbox'. Delete the solid just created. The step file is now unlocked and can be edited and saved as a rsdoc file.
Stl files can also be opened and converted into solids.