Sell them all the damned F-16s they wish to buy and get this over with.
IIRC, the issue is that Turkey has Russian S-400 SAM’s, and (since we still have and use a potload of F-16’s), we don’t want anyone doing trials of F-16’s vs. S-400’s and the data somehow getting back to the Russians.
Perhaps an acceptable compromise would be to trade some of the F-16’s for the S-400’s, or, trade some US air defense gear for the S-400’s? I think the S-400’s outrange anything we have, though.
OTOH, the Ukies have been able to beat S-400’s on occasion, so desire of the US to have a S-400 to study may be diminished?