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 under Security > Customize Profile > Time Entry. Check the Core Help for details.