I had issues with this as well. I kept receiving
PHPhotosErrorDomain error 1000
I had my photos library on external hard drives, on other servers and my home drive on another storage location. What finally worked for me was to open Settings > Security and Privacy > Full Disk Access and to add photos to that list. I could then create a new Photos Library, and after that I could finally import my old library.
jdschmidt11
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