You have to first add the domain to the hosting account (by pointing it to InfinityFree’s nameservers), then you can change them to Cloudflare’s ones and setup Cloudflare for your domain:
Now you can change the nameservers back to Cloudflare’s, remove the NS records you’ve added before on Cloudflare’s DNS page and add two A records, one with name @ and another one with name www that point to 185.27.134.215.
Remove the NS records, they’re unnecessary, and add an A record for your domain (@) that matches the IP in the client area, as mentioned in the tutorial.
Your website appears to be working now, but those DNS records still have some errors.
The FTP record should be deleted. It serves no purpose, and having Cloudflare enabled on it actually prevents any FTP access.
The MX record, both the one with type MX and and name MX don’t work, and should be deleted too. There is no mail server on that IP, and even if there was, enabling Cloudflare on anything that’s not a website name will prevent it from working.
Another thing I noticed: if that _acme-challenge subdomain is configured for our free SSL certificates, you’ll need to disable Cloudflare on it too for it to work.
As a rule, only enable Cloudflare on hostnames that you expect to only serve web pages. Cloudflare only handles web traffic and blocks everything else.