That message is letting you know that you have two elements that are connected to data within the “Projects Template” collection page, so before you can remove a specific custom field—or even an entire collection—you’ll need to disconnect those elements. This can be done on the collection page itself, or by expanding the list and clicking the Disconnect link next to the corresponding item.
If you reply with your read-only link I’d be happy to take a peek and point out the elements that are causing the error. In the meantime, you can read more about disconnecting CMS items below:
Omg, this ***problem exists from the dawn of the webflow and keeps getting more and more annoying!!! I cannot delete cms field although I deleted, unlinked and even removed the only CMS block I had on my page!!!