I do not see the control panel to start transmission

[ro481]

I do not see the control panel to start transmission

 

[Lawrence_SoCal]

Have you read the OBS Studio Quick Start Guide here (follow the Help link top right of web page)?
And hopefully you aren't referring to a separate program, like Streamlabs OBS

 

[Suslik V]

Main menu > View > Docks > Reset UI
or
Main menu > Docks > Reset UI
or
Main menu > Docks > Reset Docks
or
Main menu > View > Reset UI

Depending on the version of OBS.