dims is a keyword argument, not a positional argument (as can be seen by the preceding ; in the function signature), you want:
julia> D = mapslices(dquiv, N, dims = 2)
900×2 Matrix{Float64}:
0.0 0.0
0.0 -0.00116876
0.0 -0.00233752
(...)
See the docs here:
https://docs.julialang.org/en/v1/manual/functions/#Keyword-Arguments