We need more information. Which page from which extension?
Is this a new problem on a existing forum? What was changed recently?
If you are talking about the administration panel, login first from the forum homepage then click the admin link in the profile menu. If you try accessing the administration panel while logged out, it will not offer a login screen automatically.
It's unlikely there will be any logs for an authorization issue but see https://docs.flarum.org/troubleshoot for troubleshooting tips and how to send us your php flarum info
output.