Before we start replacing parts you should try a manual extrusion with the fan, heatsink, and drive block removed. With those components removed manually push filament into the extruder. This will allow you to confirm if there is a heating problem or not.

If you're able to extrude the issue is probably due to your drive block or drive gear. Please also make sure your extruder assembly is assembled in the proper order and your ceramic insulation tape is in good condition. I've attached a photo of the correct assembly order.

Extruder Assembly Order and Orientation.jpg