Converting FITS (NuSTAR) coordinates to WCS
You can upload your FITS files to astrometry.net (or use an API) and get the coordinates. You can optionally get new FITS files back with the coordinates included in the metadata. From there you will be able to proceed with your processing.

And, instead of uploading everything, you can also install a copy of the astrometry.net package locally if you're running Linux.