The person working with the Fortnox/Pleo integration needs a license with more user permissions. For example, the Fortnox "Bokföring" license does not grant enough access.
In Fortnox, anyone with administrators rights can go to the "manage users" (Administrera Användare) section. The solution is to change the Fortnox license of the affected user (i.e., the Pleo admin or bookkeeper). Their Fortnox license needs to be set to either "Kvitto & Utlägg” or “Bokföra” in order to work with the Pleo integration: