so when i try to load in a blueprint it say can not build this design

This may be for a lot of reasons. First it may be because you do not have all of the gamepasses that the blueprint requires. Second it may be that you do not have all the achievements that the blueprint requires. Hope this helps you 😁

One reason that happened to me is that they used a different type of cart like the plane on the junoir coaster or maybe it requires a game pass that you dont have