Foden insisted the try was good but, after a long consultation with his touch judge, Irish referee Simon McDowell ruled Nanuku had grounded the ball first.
The Fijians were beaten 31-21 in an action-packed final by reigning IRB World Series champions New Zealand.
Yet England may well be satisfied with a top-four finish after a disappointing opening day saw the new-look squad scrape into the quarter-finals on points difference only, after drawing with Canada and losing to South Africa.
But England overturned the odds on Saturday to snatch a last-gasp victory over Samoa, sealed with a late try from Ebbw Vale winger Simon Hunt, and book a semi-final against world champions Fiji.
England opened a 14-5 lead during the first half, with a penalty try and a dazzling score from Foden, before the Fijians hit back with three straight tries to move 22-14 ahead.
England responded well, with Harlequins’ Tom Guest scoring a deserved try and thought they had done enough to win it at the death.
Ryan said: “There were times we let ourselves down a bit with our performances yesterday and we were a bit embarrassed.
“I talked to the players about standards and to show they are worthy of the white shirt.
“I told them that if they don’t make it at England Sevens, they won’t make it for the Saxons or England Seniors.
“They learned some quick lessons but we are still down the pecking order and we have to work hard.”
Scotland dropped into the plate competition after losing 28-5 to Fiji in the main quarter-finals but met Samoa and went down 28-19 to the Pacific Islanders.
Wales, playing in front of new national coach Warren Gatland, opened the day with a victory over Tunisia but lost 19-12 to Australia in the semi-final of the bowl competition.