You are looking for pushfirst!. prepend!, like append!, expects multiple entries to iterate over and push into the collection.
Examples:
julia> A = Int[0]
1-element Vector{Int64}:
0
julia> push!(A, 1)
2-element Vector{Int64}:
0
1
julia> pushfirst!(A, -1)
3-element Vector{Int64}:
-1
0
1
julia> append!(A, [2, 3])
5-element Vector{Int64}:
-1
0
1
2
3
julia> prepend!(A, [-3, -2])
7-element Vector{Int64}:
-3
-2
-1
0
1
2
3