A task is Open for Time Entry and the person is staffed on the project, but you are unable to add hours against it. This is also observed when using Copy Previous Timesheet, where the resulting timesheet will exclude some of these tasks.
You can see the task in their timesheet in the New User Experience (UX), but the task is gray, so time can't be tracked on it.
Release : All Supported Releases
This can be due to additional configuration requirements not being met.
Check to verify the following is set: