First off, I apologise for being inexperience with Linux.
I’ve installed Passbolt on a VM with Linux Server, setup went fine but I only wanted to see what it was like before deploying it for the whole company so I selected not to have an SSL installed.
Now we’re looking to deploy, I would like to know if it’s possible to add an SSL to the installation we already have? It would have to be a self signed one.
Adding SSL is a matter of adding the certificates to the web server. The only change you should need to make to the passbolt configuration is to change fullBaseUrl to https://passbolt.domain.here.
Then you’ll need to configure your webserver to use your SSL certificates. Here is a decent walkthrough of configuring SSL on Nginx. Here is the documentation for Apache. You can get a certificate from Let’s Encrypt.