You have an old lens that can not tell the camera about its focal length and aperture. Example:
- lens: Tamron SP 500mm, F8 mirror built around 1980
- camera: Canon R6, file format CR3
Solution - an example...
Find all images with a focal length of "0.0 mm" in a directory and set the aperture to 8 and the focal length to 500 mm
exiftool -FNumber=8 -ApertureValue=8 -FocalLength=500.0mm -Lens=500.0mm -minFocalLength=500 -maxFocalLength=500 -if '$FocalLength eq "0.0 mm"' $dirsource