User is trying to add a memo to time entries that are already billed, but cannot do so.
This can happen if your security profile does not allow you to edit billed entries. This is the default setting for the Time and Expense security profile. In order to be able to add memos to billed entries, you need to get your security permissions customized. Consult your CORE administrator and ask to check the security bit Allow edit billed entry for you found under Settings > Access & Permissions > Security Permissions. Check CORE Help Center for details. The other workaround is to un-approve these time entries.