STL Modification

I have an stl file and i try to mod in free cad and i keep getting errors.

All i am doing is doubling the thickness of part. its 1 MM and i am doing is making it to 2 MM thick.

Depending on what the part actually is, you could load it in Cura slicer, click on the part, and on the left side there is a column of icons. The second icon from the top is scale. You can scale the whole thing or just in one particular direction. You can also specify if you want to scale by a particular percentage or to an exact size. If your part is only 1mm thick, change it there to 2mm.

The caveat in all of this is that if you have something that is dimensionally critical, like a screw hole, scaling the part in this way would change the hole from a circle to an oval.

I think you can use Meshmixer for this

When you are stressed …you lose focus on obvious. Thank you appreciate quick help. Lets see how it turns out.

I can think of a couple I use to modify still files, ThinkerCad (online import/export) or I just got the MatterControl and it works like ThinkerCad but on the PC.
I also use them to install Lettering either raised or countersunk in object I have done.
Like this lamp cover,