I am trying to get my truck to start

I am trying to get my truck to start. I have replaced the starter motor and the battery, the starter relay, and other relays, The battery was tested and is producing 12.4v. The ignition switch works. When I turn the key to turn the truck over all I get is a repeated clicking noise from the solenoid on the starter. Ive made sure all the grounds are clean and grounding. I also replaced the battery terminals. I am wondering if you know why my vechicle is having problems? I do not have a lot of money so I do not want to take this truck into a mechanic.