You're not able to add a new custom field from the 'Custom Field' tab in Space Settings ... and you're not able to add a new custom field from the Admin area Account Settings > Custom fields ... which are the two logical places to add a new custom field.
Instead, you need to click the three dots in the upper right corner and click 'Manage Custom Fields' which isn't really in a logical place.
Can you please add the ability to add new Custom Fields from the two locations mentioned above?