I am the owner of a project, and I cannot add a custom field to the project per the process described in this help page
What’s also odd is that I appear as a guest in the project members page, but am still the owner. As owner, I also cannot change guest statuses. As an owner, I should be able to control all aspects of the project.
Only org/workspace Members (logins with matching domain name) can add/edit custom fields. (As a guest you can set the value of custom fields, but no more.)