As léonie says, a Library Repair is a good idea, and you should make a backup. If you have enough room in your Mac's storage, you can just use command-D (⌘D) to duplicate your Library. It's probably named "Photos Library. photoslibrary" in your Pictures folder. When you duplicate the Library, you'll get a new one cleverly named "Photos Library copy. photoslibrary" with all the same stuff. This is just in case something messes up, and that is especially important if you are using iCloud Photos.
If you don't have enough space on your Mac's internal drive, then you'll need to use the option ( ⌥ ) key to option-drag the Library to an external drive.
If you're using an external drive, you can see the format of the drive with by selecting the drive and using the Info Window with command-i ( ⌘i )

Mine is formatted APFS, so I know that it will work with a Photos Library.
Let us know how it goes…