Julia adding packages give an "package names could not be resolved" error

Fredrik's comment should really be the answer here, so I'll expand on it a little bit:

When you do ] add SomePackage, Julia's package manager will search the General Registry to look for the package, and then install it. As you will see when looking at the M entry in the general registry, MLPlots is not registered, hence the "not found in ... registry" error you're seeing.

Now it should be noted that packages don't have to be installed in the general registry to be installed - the general registry is just the default registry, and largely a convenience tool in the Julia ecosystem which provides the default location to check for packages and enforces certain standards on registered packages to ensure things play nicely with each other. That being said, nothing prevents you from

1. Adding a different registry: as noted in the Pkg.jl docs introduction, "it allows multiple registries managed by different parties to interact seamlessly. In particular, this includes private registries which can live behind corporate firewalls. One example of this is the JuliaPro registry, which offers a curated subset of the General registry;

2. Just adding a package that's not registered anywhere, either from a local path or using a GitHub (or equivalent service, e.g. GitLab) link.

While the second would in principle be an option in this case, at this point the other problem mentioned in Fredrik's comment becomes pertinent:

(@v1.6) pkg> activate --temp
  Activating new environment at `/tmp/jl_pKGogE/Project.toml`

(jl_pKGogE) pkg> add https://github.com/JuliaML/MLPlots.jl
     Cloning git-repo `https://github.com/JuliaML/MLPlots.jl`
    Updating git-repo `https://github.com/JuliaML/MLPlots.jl`
ERROR: could not find project file in package at `https://github.com/JuliaML/MLPlots.jl` maybe `subdir` needs to be specified

Since v1.0, Julia requires packages to come with a "project" file (Project.toml), which specifies the packages dependencies including (at least as best practice, and for packages in the general registry) compatibility bounds. As the package you're looking at was last worked on 5 years ago, it is not in a format that Julia's current package manager can deal with.

Looking at the package itself I would also say that this isn't necessarily a problem - not trying to knock Tom here who's done exceptional and pioneering work in the Julia plotting ecosystem, but this specific package seems to be a bit of a proof-of-concept which doesn't provide an awful lot of functionality. Maybe if you post on the Julia Discourse a bit of a more open-ended request for help with whatever problem you're trying to solve people can point you towards the current state-of-the-art packages in the ecosystem.

I'll just add here that the central package for standard machine learning in Julia at the moment is probably MLJ.jl, which includes a roc_curve function that could give you the basis for the ROCAnalysis plot you see in the MLPlots readme.