Cannot use or have problems using Let's Encrypt or sslforfree.com
Many people using free hosting also want free SSL certificates. A new, popular provider of free SSL certificates is a service called Let's Encrypt. There are various tools to get Let's Encrypt certificates, sslforfree.com is a popular provider. sslforfree.com is simply a website built around Let's Encrypt, but uses Let's Encrypt to verify the domain names and create the certificates.
It is not possible to use Let's Encrypt certificates (and sslforfree.com by extension) on InfinityFree. Let's Encrypt verifies that you own a domain name by checking a URL on your website for a certain value. However, all InfinityFree accounts have a security system which prevents access to most automated bots and programs, and only lets valid web browsers access your website. While this is perfectly fine for website hosting, it does make the Let's Encrypt verification tool unable to check whether your own the domain.
Let's Encrypt also offers TXT record validation. However, you cannot set TXT records on domains hosted by InfinityFree. If you are using your own nameservers, you may be able to set TXT records and get your Let's Encrypt certificate. These certificates can be installed on InfinityFree without any problems.
However, Comodo Instant SSL does offer a Free SSL program which uses email addresses or CNAME DNS records to verify domain ownership, which can be used with InfinityFree. You can learn more and sign up here: https://www.instantssl.com/free-ssl-certificate.html