Can't Add from Tab P11 Pro

I'm starting a Calibre installation (under Win10) from scratch, and having almost no luck with the process.

I use ReadEra and Moon+ Reader Pro (migrating to ReadEra, preferred over Moon+ Reader). The files are stored in the folder Book, residing on the P11's SD card.

I can place a file (PDF or EPUB) on the P11's Book folder, as hoped for. I can't bring a file into the Calibre library.

The device recognized by Calibre, as well as my Win10 laptop.