So i got an 03' 2500HD gmc sierra 6.0

This is my issue-
When I turn the key, nothing at all happens, Lights turn on but starter wont tick.
The battery is good, fully charged.
I checked all fuses and connections, all is well.
The starter does work, I brought to napa to test, and works perfectly.
The starter won't budge. I checked for power, and it seems to be getting power.
IS there anything that I am missing?