We do have an SSL certification ,and always had..
https://i.postimg.cc/VLgtxZqB/55677.png
https://i.imgur.com/qna2oIy.png
https://www.sslshopper.com/ssl-checker.html#hostname=https://bits.do
I guess that its a local problem on your side?
I'd suggest you to scan your PC
First, your site looks nice at first sight but I didn't register in it yet as I'm also getting the SSL certification error so it's not just for him and I have tried with two machines including the virtual machine from another country so you need to fix it as soon as possible as it may lead to security concern
I hope this will solve the issue ,although i haven't faced it by myself ,and only 2 people have faced it so far (as far as i know)..