Most of the times this occurs when the project status is (still) set to estimate. 

When using Rodeo you can only write hours or book costs to a project when the status is set to active. 

Rodeo uses 4 different kinds of project status:

The estimate for this project is created and send to the client.
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​2. Active:
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The project is accepted and operational. Hours can be tracked and costs can be booked.
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​3. Completed:
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The project is completed an delivered. Hours can no longer be tracked but you still expect some invoices.
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​4. Closed:
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Project is completed and closed. Hours can no longer be tracked and costs no longer booked.

Another possibility is that there is no budget created for the project. There must be a project activity based on time before hours van be tracked.

Simply add a budget activity and you and your team are able to write hours.