An expense in Pleo has a category that requires a project-specific tag, but doesn't have one.
There are two alternative solutions:
Remove the requirement of a project tag (in Fortnox)
In Fortnox, go to "Register".
Go to "Chart of Accounts" (Kontoplan).
Click on the specific account (Konto) a.k.a. Pleo category.
Under “Project”, change from "mandatory" (Obligatorisk) to either "allowed" or "not allowed":
Click "Save" (Spara).
In Pleo, add a relevant project tag to the expense(s) in question.