In Julia, max is for comparison of two elements and maximum is for the highest value in an iterable. Here, you want comparison between two elements to be broadcasted over every value of your array, so you can do it as follows:
julia> a = rand(10)
10-element Vector{Float64}:
0.22992278012202094
0.2474774269332295
0.027469463935002603
0.2212289112594984
0.6776369063933763
0.560465219340989
0.7627282729774861
0.33638924323772235
0.8933858558943317
0.5810967639065461
julia> max.(a, 0.5)
10-element Vector{Float64}:
0.5
0.5
0.5
0.5
0.6776369063933763
0.560465219340989
0.7627282729774861
0.5
0.8933858558943317
0.5810967639065461