Generally, if there are no cameras rendering to the Game view this is unwanted behaviour, so there is an Editor-only warning to help debug this situation.
Right-click on the Game view tab and uncheck warn if no cameras rendering to remove the warning.
For the warning, a camera must be active and enabled to be considered rendering. Rendering a camera via code does not count. If you are rendering cameras manually, see above.
Unity, huh, how?