How am I suppose to transfer my photos library to Photos?

If the recommendation by FoxFifth should not be working as expected, make sure your external hard drive is compatible with Photos. Check, if the external device has been prepared as described here: Move your Photos library to save space on your Mac - Apple Support

The hard drive must not have been used for Time Machine backups, the file system format needs to be MacOS Extended (Journaled) or APFS, and the drive needs a wired connection.

Then Photos should be able to migrate the iPhoto Library, unless the library is from a very early version of iPhoto.

Hold down the option key while opening the Photos app and it will give you the option to select your iPhoto library and will open and convert it.