You haven't shared your file, so it is impossible to know the exact problem.
However, the closest possible to an answer is that the UVs are not unwrapped, so all you can see is a black preview.
To unwrap it, select your model, go to
Edit Mode, and select all by pressing
A. Then press
Smart UV Project if you want to do it automatically without adding seams.
If that doesn't work, you can select an
Edge Loop by pressing
LMB and press
Mark Seam. Then, after adding the seams to the appropriate places, select all and press
If you don't know UV Unwrapping, I recommend you search up some of the thousands of tutorials on them on YouTube.