You cannot add or delete anything on one device model in OneClick because it is locked by a user (see message in title above).
There is a possibility to unlock models over the OneClick Web Page. Please be advised that all the changes done by the user that locked this model are gone afterwards. So make sure you check with the "locking" user, before you go ahead with the following solution:
- Access the OneClick Web Page (as an Administrator user) --> Administration --> Debugging --> Model Locks
- There will be entries showing the locked models.
- The lock can be forcibly "removed" from the table listing.
Please see related Tech Doc - TEC523462 - Spectrum - Model Lock Error while trying to discover the devices manually after migration