Dungeon Master Guide 5.5e

I recently got a book and now whenever I go to items or magical items they say I still need to purchase the book, I tried resyncing but no luck

Can you read the book, or is that also blocked?

Where are the item links? Are you sure they're not linking to the old version of the items? (Anything published before 2024 will link to the 5.0 versions.)

Does the book appear in your purchase history in the marketplace? If it doesn't, were you billed for it?

Ultimately, you may well have to contact customer support to get it untangled.