Cannot open discourse after installation

I finally succeeded to install discourse but cannot log in

it said discourse is now installed but cannot open

Any solutions?

I think this is because the wrong DNS record

like this picture I set my DNS A type record

the thing is I cannot put more than 1 dot in host. So I put ‘forum’ in host and put my instance IP in value.

Is it the wrong way?

Not sure what the issue might be, but you should follow Discourse official Standard Installation rather than the Digital-Ocean one-click (or whatever else you might have done?)