Keyword parameters (i.e. arguments appearing after the semi-colon “;”) have to be named, which is not the case of dims here, for which you directly put the value 2.
This should work:
julia> data = rand(1000,1000);
julia> using Statistics
julia> result = std(data; corrected=true, mean=nothing, dims=2)
1000×1 Array{Float64,2}:
0.27789883476970717
0.2834628928652056
0.2843101448288627
0.290594711126601
...
or, since it looks like you want to keep the default values for corrected and mean:
Hopefully, you’ve got your answer, but in the future, please provide a minimal working example that demonstrates your problem and shows the error message. Please read: make it easier to help you