When user is reviewing time entries and tries to move entries from one project phase to another, Core says they are locked. These hours have not been billed and the Global Settings value for "Time Entry should not be saved/deleted if older than" is greater than the age of the time entries. Why are the time entries locked?
You or someone else in your company has the Invoices screen open, causing those time entries to be locked for billing. Cancel the billing process and exit the Invoices screen. Refresh your screen and you should be able to move them from one phase to another now.