using Symbolics
@variables u::Complex # or any other type

gives the following error:

ERROR: LoadError: AssertionError: @variables expects a tuple of expressions or an expression of a tuple (`@variables x y z(t) v[1:3] w[1:2,1:4]` or `@variables x y z(t) v[1:3] w[1:2,1:4] k=1.0`)

Seems to work for me, what version of Symbolics do you have installed?

julia> @variables x::Real z::Complex{Real} (X::Real)[1:10, 1:10] (Z::Complex{Real})[1:10] s::String
5-element Vector{Any}:
x
z
X[1:10,1:10]
Z[1:10]
s
(jl_hqZDlM) pkg> st
Status `/tmp/jl_hqZDlM/Project.toml`
[0c5d862f] Symbolics v1.4.2