Engine won't crank and just clicks when you turn the key?

I am working on a 99 Blazer 4.3 l vortec 4 wheel drive.
When I go to turn the key it just clicks. Put a brand new
starter in it check the battery battery is good. Checked all
ground wires and all wires and they are good. I can turn
the motor over by hand a full rotation. When I get back in
the car and turn them with the key it just clicks every once
in awhile it'll partially turn but then I'll just go back to
clicking. What is causing it just to click. I just replaced the
intake manifold gaskets and once everything was put
back together properly and want to start the vehicle it just
clicks can somebody please help me.