You need low availability, which is probably very limited at least through the end of the summer due to revenge travel and DL's limited schedules at this time.

The DL forum has information about what numbers of miles would be considered low on different types of routes (although this doesn't guarantee that DL has made award inventory available to even close partners such as AF/KLM). Assuming that you don't need an account or some RDMs to search for awards on delta.dumb, you could try looking at the award calendar on the DL website to find dates that *might* have availability for you.

Also, AFAIK DL uses married segment logic for award availability, so you might do better if you're booking as part of a TATL award on AF/KLM or DL metal across the pond.