Actually there was another problem with the HiddenMarkovModels.jl package, which I reported here.
I used the add command but get a different error now
julia> Pkg.add(url="https://github.com/ahwillia/HiddenMarkovModel.jl")
Cloning git-repo `https://github.com/ahwillia/HiddenMarkovModel.jl`
Updating git-repo `https://github.com/ahwillia/HiddenMarkovModel.jl`
ERROR: could not find project file in package at `https://github.com/ahwillia/HiddenMarkovModel.jl` maybe `subdir` needs to be specified
Stacktrace:
[1] pkgerror(::String, ::Vararg{String,N} where N) at C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.5\Pkg\src\Types.jl:52
[2] resolve_projectfile!(::Pkg.Types.Context, ::Pkg.Types.PackageSpec, ::String) at C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.5\Pkg\src\Types.jl:672
[3] (::Pkg.Types.var"#49#50"{Pkg.Types.Context,Pkg.Types.PackageSpec})(::LibGit2.GitRepo) at C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.5\Pkg\src\Types.jl:645
[4] with(::Pkg.Types.var"#49#50"{Pkg.Types.Context,Pkg.Types.PackageSpec}, ::LibGit2.GitRepo) at C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.5\LibGit2\src\types.jl:1128
[5] handle_repo_add!(::Pkg.Types.Context, ::Pkg.Types.PackageSpec) at C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.5\Pkg\src\Types.jl:597
[6] handle_repos_add!(::Pkg.Types.Context, ::Array{Pkg.Types.PackageSpec,1}) at C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.5\Pkg\src\Types.jl:663
[7] add(::Pkg.Types.Context, ::Array{Pkg.Types.PackageSpec,1}; preserve::Pkg.Types.PreserveLevel, platform::Pkg.BinaryPlatforms.Windows, kwargs::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}) at C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.5\Pkg\src\API.jl:168
[8] add(::Pkg.Types.Context, ::Array{Pkg.Types.PackageSpec,1}) at C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.5\Pkg\src\API.jl:139
[9] #add#21 at C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.5\Pkg\src\API.jl:67 [inlined]
[10] add at C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.5\Pkg\src\API.jl:67 [inlined]
[11] #add#19 at C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.5\Pkg\src\API.jl:65 [inlined]
[12] add at C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.5\Pkg\src\API.jl:65 [inlined]
[13] add(; name::Nothing, uuid::Nothing, version::Nothing, url::String, rev::Nothing, path::Nothing, mode::Pkg.Types.PackageMode, subdir::Nothing, kwargs::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}) at C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.5\Pkg\src\API.jl:82
[14] top-level scope at REPL[7]:1