To modify a protected cell or chart first remove protection using Unprotect Sheet command
You may receive this error message if the current cell or chart is protected by password.
To unprotected go to Review Tab / Changes group. After that you must insert the password.
Error class: Office; The cell or chart that you are trying to change is protected and therefore read-only.
Error type: The cell or chart that you are trying to change is protected and therefore read-only.
Error message: The cell or chart that you are trying to change is protected and herefore read-only.