takjas: Try something: move d4-d6-d8, then move your cursor over each square on the board to see if the arrow becomes a hand. It could be a square which doesn't make sense. If it becomes a hand just go there.
Anyway it's probably just a bug in the game, which is too bad because Fencer no longer corrects them. You'd better resign, otherwise this game will just eat away your vacation days.
Look at the rules, section Dameo: When a pawn reaches the last row, it promotes to a queen. However, if a pawn reaches the last row while jumping and can jump more pieces backwards, it does not promote and continues jumping as a pawn.
Justaminute: Game rules -> Dameo. The page lists all the quirks for particular variants of checkers. For Dameo (the last) "Pawns can jump backwards" is checked, so yes, for this particular variant they do.