You installed Manager on a subdomain of https://larebco.com/ which already correctly has an SSL Certificate installed. Are manager and the main domain on the same VPS? Also note from installation instructions that “…This will involve setting up A record on your domain so the domain name (or subdomain) points to IP address of your VPS (it says in instructions Internet router)…”
You can not have exactly done as prescribed at Ubuntu | Manager because you would have at least changed the folder.
i am using manager on VPS for last 5 years never faced such issue.
you should use https://manager.larebco.com/ without port number, it is showing Self sign cert issue.
Did you try to reissue certificate? just find and delete certificate on vps, on next attemp to open https://manager.larebco.com/ it will create new certificate.
i am also using it on subdomain without any problem
edit: i just checked, your link is opening Webuzo. contol panel installed on VPS. For accounting software you use separate VPS and install only manager related material on it.