It’s possible that P4V has guided you to create a stream depot and a mainline, but has somehow created a non-stream workspace for you. From what I’ve heard, that’s likely to happen for first-time P4V users due to something in the setup.
To see if that’s the problem, go to Connection/Edit Workspace and look in the ‘Stream’ field. Is it empty? If so, use Browse to select the stream. This will turn your current workspace into a stream workspace.