Apple has a restriction on macOS, to allow Backup Intelligence to read your files, you need to allow Full Disk Access for Backup Intelligence, this can be done by:
A pop up window will appear and ask you to allow full disk access. Click on "Security & Privacy"
You need to click the lock to make changes, and then enter in your administrator password for your computer
On the left scroll down until you see Full Disk Access, then click on Full Disk Access
Then click +
Add in Backup Intelligence and make sure it's got a tick beside it, then click the lock to save the change.