Can you elaborate more, maybe with pictures? Intel or AMD?
I don't remember having much issue installing the pump block, and it would be helpful to know in what way you are having trouble? I have a Ryzen system, and mine only installed with the metal bracket with 4 screw holes on it. There was a plastic ring with four tabs on it that I didn't need, maybe it's for intel systems or maybe it's just for the packaging.
Wow there is definitely a lack of documentation for this product, but Asetek lists it as an option part for gen4 pumps, I suspect it's just "part" of the installation kit and not needed for your 6th gen pump. Mine installed securely without it if that helps.
Also I finally found this installation instructions on Asetek site, they have a note the Ring is only needed for Gen4 pumps, since the 645LT is a Gen6 pump, I'm pretty sure you can just toss that retention ring.
"Step 3 Secure retention ring to the pump / block combo. Attach from the side that will contact the CPU. Swivel and click into place. NOTE: For Asetek Gen4 pumps, use the plastic retention clip to secure the pump to the retention ring after it is placed in the desired orientation"