I have attempted to create a monster that has the ability to cast Toll the Dead.  When I add  Toll the Dead into the Actions Description, it says that I need to purchase Xanathar's Guide when I mouse over the spell.  However, I have purchased the 2024 PHB, so I should have access to that spell.

There are two totally separate versions of that spell on D&D Beyond, one from Xanathar's Guide to Everything and one from the 2024 Player's Handbook. It sounds like you've added the former, which you don't have access to. You'll need to remove it and add the other one (the one under Core Rules that doesn't say "legacy") instead.

Thank you!