Searching for D1 award or revenue fares? Open jaw on EU end is not permitted on EU origin revenue fares (unless the open jaw is within the same country). Believe the award search will look for both award and miles+cash options (miles+cash are just awards paid partially with cash instead of all miles -- they are also a ripoff as DL is charging you 1.9 CPM to make up the mileage difference). However, miles+cash option requires all DL metal and non-DL metal options may be causing search to error out. Try searching for Main awards instead (which will also show D1 options). This is working for me.

Note that EU origin awards will have significant carrier-imposed surcharges applied to them (ex-US awards are not subject to these). Ex-UK flights also get hit with with UK APD tax (currently £194.00 for business class), so it's a double whammy. Don't expect to find any D1 deals to/from BNA as D1 awards are now dynamic (largely fare based) to/from US and Canada (there are also no longer separate partner award levels to/from US/Canada).