When reviewing time entries and trying to move some 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?
It seems that you or someone else in your company has the Invoices screen open and so those time entries are locked for billing. You need to 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.