Can you not use Letsencrypt locally? I think I actually saw something about that recently here on HN. What is the difference between this and LE if true? I wo

Can you not use Letsencrypt locally? I think I actually saw something about that recently here on HN. What is the difference between this and LE if true? I would like to use a .test domain. I use that currently and just click "proceed anyway" whenever the warning pops up and It's pretty often lately.