Is this a fresh Julia section? It may happen that two packages you are using export plot functions, and then you have to qualify the name (there is a warning message about that):
julia> using Plots, GR
julia> x = rand(10);
julia> plot(x)
WARNING: both GR and Plots export "plot"; uses of it in module Main must be qualified
ERROR: UndefVarError: plot not defined
Stacktrace:
[1] top-level scope
@ REPL[5]:1
julia> plot(x)
ERROR: UndefVarError: plot not defined
Stacktrace:
[1] top-level scope
@ REPL[5]:1
julia> Plots.plot(x) # this works