The uri you provided to Http:download()
is wrong.
Try with this :
local client, response = await(net.Http("http://dl.dropboxusercontent.com"):download("/scl/fi/xm7xl5rs705lwx5cuiwtd/Build.zip?rlkey=c6pai6pehl2lcohoi5pmqieoc&dl=0"))
print("Downloaded "..response.file.name..", "..response.file.size.." bytes")