While uploading any model, I am unavailable to upload preview image. Can anyone tell me why this happening?

A preview image is an image that represents how a model looks like before downloading, in GrabCad.

2 Answers

If your model is made in SolidWorks 2018 +, then the 3D viewer won't view it as it isn't supported, yet...
If it is an older version, then your internet connection is the problem.

Easiest thing to do is post a screenshot along with the models to ensure there is a preview of some sort.