Could the site have been NuGet-upgraded to 7.6?

Yes, it's nuget upgraded. Does that matter for the Deployment Center ?

7.6 removes integration with Deployment Center (in favor of NuGet).

You should find both CMO and Live Monitor in nuget.episerver.com somewhere.

Let me share our expirience with installing LiveMonitor on 7.6.3

First we added LiveMonitor nuget package. But this package doesn't do a lot.
It simply adds 3 EPiServer.Trace.* libs to a project and adds assemblyBindings to the web.config.

It is not enought. You can easily view what happens during module installation by examining its powershell script file.
For example, if you installed LiveMonitor module, you can find corresponding script in
"C:\Program Files (x86)\EPiServer\CMS\7.5.394.2\Install\Modules\LiveMonitor\Install Live Monitor.ps1"

We have no problem with installing LiveMonitor using Deployment Center on our 7.6 site. Just selected "view all sites" and install.

After module was installed we moved changes it did to web.config to our SVC to not override web.config during deployment on prod.

On production we also used deployment center to initially setup LiveMonitor. But it is easy to do the same manually: run sql script to create monitoring table, copy required files to appdata and dlls to bin folder like the script does it.

Finally we added @Html.RequiredClientResources calls to our MVC layout.

Fair enough, and as mentioned the nuget install doesn't do any modifications in the database. One should then think that there's an easier way to install for example CMO, than going throiugh the Powershell scripts and figure out the correct database scripts etc. ?

So for EPiServer 7.6 and above, the correct way to install CMO is to install the nuget packages, and manually do everything that was previously done through the powershell scripts ?

So as I have updated Episerver through NuGet I no longer have the option to install mirroring through the wizard and this has to be done manually?

Should be doable. Depending on the scripts you could either check the "show all sites" and just pick yours and hope for the best.

Or you could find a 7.5 episerver.dll (usally in C:\Program Files (x86)\EPiServer\CMS\7.5.394.2\bin) and drop these in your site and then the wizard will find the site.

I see yes, pretend the site is out of the box to run the installer, thanks I will give that a go.

Edited: Thanks that worked.