when using mask in Lightroom it tells me that there are no prople in the image. The image is a portrait.

Does it happen with every portrait or just this one
If you try another photo with a clear face and good lighting does People masking still say no people found, If it happens on every image then it sounds more like Lightroom isnt detecting people correctly rather than an issue with that specific photo

Please do the LR menu command Help > System Info, click the Copy button in the upper right corner, and paste into your reply here, so we can see exactly which versions of hardware and software LR thinks you're running and important LR options that are set.  That will let us provide an exact answer without guessing.