Kikr core: cassete keeps spinning when I stop cycling and chain loses tension

Hi!

I’ve recently bought a kickr core second hand and it came without a cassette and the freehub was not mounted (the previous owner had a SRAM cassette).

I assembled everything with a shimano 10 speed cassette following the instruction videos. But when I stop spinning the pedals, the cassette keeps moving for a short while, making the top of the chain become loose.

What could the issue be? Or is there a step-by-step plan that I can follow to troubleshoot?

Thanks!