Adding something like sub.psl.tld will throw an error, you can olny add psl.tod.
That’s not how you do it. Now you have mixed nameservers at the root level. Use the CNAME method if you are using Cloudflare. Also, not relevant to the error message you are receiving.
InfinityFree has two options for adding your domain to your hosting account: either by changing your nameservers (not NS records) or by validating with CNAME. Since you are using Cloudflare, you will need to validate your domain using the CNAME method.