The error "You have reached your quota for the requested resource. (Code: 2005)" is thrown when trying to upload a new Custom Certificate or edit an existing one.
Usually this error is due to the fact that the quota for Custom Certificate is by type of certificate: Custom Legacy vs Custom Modern. By default when uploading a certificate, the type is set to Custom Modern. But if you only have quota for Custom Legacy certificates then the error will be thrown.
First of all check your entitlements under SSL/TLS -> Edge Certificate. It will read:You can upload X Custom Legacy certificate pack. You can upload Y Custom Modern certificate packs.Then when actually uploading or editing the certificate, make sure you select the appropriate option in the Legacy Client Support dropdown at the bottom.
More details about Custom Certificates can be found here: