The credential store was created with "Password protected" option enabled and locked with a password.
To use or modify it one have to unlock it with this password.
On the webuUI under "Policies -> Credential Stores" at the closed Credential Store there is a warning message:
Please open the credential store to modify or delete it!
On the main menu at LHS please click "Unlock credential store"
Please find the locked Credential Store and open it with the correct password.
1) Select credential store to unlock:
2) Enter unlock password: