Why cannot DBA clear named credentials in 13c OEM?

Some DBAs report that they cannot clear unwanted preferred named credentials listed in "My Preferences" tab in OEM Security anymore.

They also cannot clear targets under "Target Preferred Credentials". They got the error of "Named credential association not removed, nothing to clear."

Any idea why this happens and what the error means and what to be to make them able to remove unwanted targets? Please advise.

Thanks, Sarah W.