Rulings
Q1: I tried to delete 1 of my other Digimon using this card's inherited effect when an opponent's Digimon attacked, but I couldn't delete the other Digimon due to another effect. Can I end that attack at such times?
No, you can't end that attack. If the "by doing X" condition isn't met, the rest of the effect isn't processed. In this case, if you can't delete 1 of your other Digimon, then you can't end the attack.
Q2: What does "end that attack" mean, exactly?
After this effect activates, the current timing makes a transition to the end of attack timing. For example, if this effect activates during the attack declaration timing, it will make a transition to the end of attack timing. A transition to the counter timing or block timing won't occur, and the attack won't succeed.
Q3: Can an "end that attack" effect end an attack by a Digimon that isn't affected by effects?
Yes, you can end such attacks. "End that attack" effects are effects that change the timing, they don't affect an attacking Digimon.
Data Sources
Lv.4 BlackGatomon EX7-054 U Champion | Virus | Dark Animal [When Digivolving] [On Deletion] By trashing 1 card in your hand, 1 of your Digimon gains <Blocker> and <Retaliation> until the end of your opponent's turn. --- Inherited: [Opponent's Turn] [Once Per Turn] When any of your opponent's Digimon attack, by deleting 1 of your other Digimon, end that attack.
Lv.4 BlackGatomon EX7-054 U Champion | Virus | Dark Animal [When Digivolving] [On Deletion] By trashing 1 card in your hand, 1 of your Digimon gains <Blocker> and <Retaliation> until the end of your opponent's turn. --- Inherited: [Opponent's Turn] [Once Per Turn] When any of your opponent's Digimon attack, by deleting 1 of your other Digimon, end that attack.
Lv.4 BlackGatomon EX7-054 U Champion | Virus | Dark Animal [When Digivolving] [On Deletion] By trashing 1 card in your hand, 1 of your Digimon gains <Blocker> and <Retaliation> until the end of your opponent's turn. --- Inherited: [Opponent's Turn] [Once Per Turn] When an opponent's Digimon attacks, by deleting 1 of your other Digimon, end the attack.
Lv.4 ブラックテイルモン EX7-054 U 成熟期 | ウィルス種 | 魔獣型 【進化時】【消滅時】自分の手札1枚を破棄することで、相手のターン終了まで、自分のデジモン1体は≪ブロッカー≫と≪道連れ≫を得る。 --- Inherited: 【相手のターン】[ターンに1回]相手のデジモンがアタックしたとき、他の自分のデジモン1体を消滅させることで、そのアタックを終了する。