Why couldn't I win in this situation with ron?
In order to win with ron or tsumo, your hand needs to have a yaku, in addition to having four sets and a pair (except for 7 pairs and 13 orphans hands). You can think of yaku as a specific pattern in the hand or a specific way to complete the hand. For example, tanyao requires that all of your tiles are numbered tiles between 2-8 (inclusive). Dora does not count as yaku.

Not directly related but you might run into this issue sooner or later: another requirement for winning with ron is that you must not be in furiten. There are three types of furiten and they are a bit complicated, so I will let you read about it from your favorite rule guide. Note that furiten never affects your ability to win by tsumo.