Unable to locate package referenceassemblies-pcl

I am on Ubuntu and I am entering the following command in my terminal:

sudo apt-get install -y binutils mono-complete ca-certificates-mono referenceassemblies-pcl fsharp

This fails, giving the following error:

E: Unable to locate package referenceassemblies-pcl

I have googled this error and found some people suggesting to make the referenceassemblies-pcl available by doing the following, which intuitively makes sense:

sudo apt-add-repository ppa:referenceassemblies-pci/ppa

But this gives the following error:

Cannot add PPA: 'ppa:~referenceassemblies-pci/ubuntu/ppa'.ERROR: '~referenceassemblies-pci' user or team does not exist.

How can I get my initial command to work? I presume I need to install/download referenceassemblies-pcl somehow but I can't find anywhere explaining how to do this that works for me.