If your domain is registered with Cloudflare then you can’t change the nameservers. It is part of the terms and services and a message when you are buying a domain.
You need to move your domain off Cloudflare to change your nameservers.
Most hosting does not need you to set their nameservers to use their service. I have never used Hostinger, so I have no idea where you want to look in your Hostinger account for the IP addresses that you need to know on order to create your DNS records at Cloudflare.
Their online KB did not yield relevant results the last time I searched for this documentation. If you find it, maybe you could share a link to it here in the Community so we can use it to help others who want to use Hostinger with their Cloudflare account.