.TK domains are free domains that I think kind of have a reputation of being used for sketchy web sites. I like to sign up for these free domains when I'm testing things. It's nice to have a free throwaway domain for testing purposes.
But I've seen several instances of people wondering if .tk domains can be used with LetsEncrypt free SSL certificates. Some people seemed sure that you can't get a free certificate from Letsencrypt for a .tk domain.
So I tested it. I signed up for my free .tk domain and I created a virtual server in Virtualmin for it and then I requested a certificate from Letsencrypt usiing the built in Virtualmin module for that. The result was that it worked.
So yes, the free certificates from LetsEncrypt will work with the free domains from .tk.