I’ve just checked on our end and this is currently a known issue, where some merchants might not be able to change permissions for already created collaborator accounts. Our developers are aware of this and are taking a closer look at it as we speak. As a workaround, you can delete the collaborator account and get the collaborator to resubmit their request for access to your store. From there you’ll need to add them with all the permissions you’ll want to provide.
We’re sorry for any inconveniences caused. Thanks for your patience and understanding on this matter!