Renege and misdeal
A renege is not following suit when you could; a misdeal is a deal gone wrong. Both happen only at a real table, and the three sources leave the penalty to the table.
Diamonds were led. Your J♦ is the left bower, a heart this hand, so it cannot follow. Your 10♦ is your only diamond, and you must play it.
Play the 10♦What is a renege?
Not following suit when you could: playing a card of another suit while you hold a card of the suit led. The rule it breaks is in all three sources: everyone must follow suit if able. Remember that the left bower belongs to trump, not to its printed suit.
What is the penalty for a renege?
pagat.com, Bicycle and the Semi-Official page give none. League rulebooks do: Ohio’s tournament rules, the World Euchre Championship and the City of Plymouth in Michigan give the other side 2 points for a renege, 4 when it happens against a loner. At a home table, agree on it before the first deal.
Can you renege on purpose?
No. Following suit is a rule, not a choice: a card of the suit led must be played if you have one. Only a player with none of that suit may trump or throw any card.
Do you have to trump when you cannot follow suit?
No. With none of the suit led you may play any card: a trump to try to take the trick, or another card to keep your trumps (pagat.com, Bicycle, the Semi-Official page).
What is a misdeal?
A deal gone wrong: a player with the wrong number of cards, or a deal that breaks the pattern. Bicycle: the dealer keeps to the pattern started with, three then two or two then three. The Semi-Official page: after the deal there are four cards left over. None of the three sets a penalty or a remedy, so agree on one before the game.
What if a card is turned over in the deal?
None of the three sources rules on it. Settle it with your table before you play.
In the app
Neither can happen in the app. Every hand is dealt from a sealed shuffle, and the app offers only the cards you may play; with Show playable cards on in Settings, the others are dimmed. At a real table, the rules above are what to agree on.
Sources: pagat.com, The Play; Bicycle, The Deal and The Play; the Semi-Official Euchre Home Page, The Deal and The Play; the league rulebooks of Ohio, the World Euchre Championship and Plymouth, Michigan.