Hi!
Unfortunately your code sample is not reproducible on my computer since I don’t have the underlying file. However, it seems like the method spring_layout does exist for SimpleGraph{Int}:
julia> using Graphs, GraphPlot
julia> hasmethod(spring_layout, Tuple{SimpleGraph{Int}})
true
Have you tried restarting your REPL? It sometimes happens to me that a type is “imported twice” and some packages try to use the older version.
If that doesn’t work, can you update your packages and try again?