When I play on the Steam Deck (and my son on his Steam Deck) it is not possible to insert a unique item into the Egg at Nemesis (that replace button where you place a 0 LP/ 0 WW unique to empower it). This only works on PC with a mouse…
Just talked with a fellow in tg, who is playing on steamdeck about egg issue (someone asked Mike on a devs stream)
It is possible. Open egg, open bag, select item with deck’s stick…
Tap on a screen with your finger inside egg
Confirm and select button to fight nemesis