I tried to go to the admin panel of your site, and got a
404 Not Found error. Is the
frontName of your admin panel something other than
If you don't know the admin panel front name, you can see what it is in the
app/etc/env.php file. Somewhere in that file, there should be something like this:
'backend' => [
'frontName' => 'admin'
Do you have access to the server logs (nginx or Apache)? They may have an indication of what may be going on. Perhaps the admin virtualhost is not configured properly, or there's some other generic error occurring that is being logged with the HTTP server.
I'd also look in your Magento
var/log/exception.log to see if any information shows up there as to errors within Magento itself that are contributing to this.
And finally, if you have this routed through a firewall or load balancer, you could have crossed wires in how things are connected or a disconnected admin route.
As for the SSL issue, if you're not running this in production, you can easily just run the store under non-SSL for testing / development purposes. If you need a low-cost (or free) certificate for testing / development, I would suggest you look into Let's Encrypt. If you have SSH access to the server, you install a tool, run a command and it can take care of everything for you (generally).