Pact of the Tome warlock, Book of Ancient secrets invocation, Galdur's tower.

As stated in the title I have a 12th-level warlock (pact of the tome) with the Book of Ancient Secrets invocation. In a recent adventure, I acquired a scroll of Galdur's Tower (wizard spell, level 3, ritual tag). 

I went into my character sheet, clicked noncore DND content ON, and then went to the Class tab to add the spell. It does not appear even though I meet the prerequisites and other spells are listed as available.

Is this a bug for this spell? Are there any other ways to make it appear as an option for my character?

Thanks!

Book of Ancient Secrets only lets you add Ritual spells. 

You can now inscribe magical rituals in your Book of Shadows. Choose two 1st-level spells that have the ritual tag from any class’s spell list (the two needn’t be from the same list).

On your adventures, you can add other ritual spells to your Book of Shadows.

Galder's Tower is not a Ritual spell.

Thank you.