Why would Mussolini attack Ethiopia in the 1920s? As Luke states, the pacification of Italian Libya is still ongoing - I don't know much of the Italian economy during the 20s - and I'm pretty sure Mussolini focused on consolidating his hold over the Italian government. The Italians aren't in any position to launch an invasion of Ethiopia when their resources are going towards development of Italian Libya, Eritrea and Somalia or preparing for a future war in Ethiopia - I believe the Italians had begun planning for an invasion of Ethiopia by the late 20s or early 30s. So, why would the Italians invade when they have other matters to tend to?
Should the Italians invade in the 1920s, the League of Nations could do something about it (unlike IOTL) and enact effective economic sanctions which include coal and oil, along with closing the Suez Canal being closed to Italian shipping which is then forced to go through Gibraltar and the long way around Africa. The thing that interests me is whether or not the Ethiopians are going to able to win during the 20s as they have problems of their own - the conservative reactionary Zawditu is Empress and unwilling to modernize Ethiopia whereas Ras Tafari is attempting to implement reforms, being unsuccessful in convincing her and the nobility to pass many of these reforms. The Ethiopians may receive support from France who had been pro-Ethiopia in the past, assisting in lifting the arms embargo that Lij Iyasu had caused and even in providing support as far back as 1894-96, the First Italo-Ethiopian War. The thing is, the French (and by extension, the British) have no reason to support Mussolini as Hitler hasn't come to power and a rearming Germany isn't a threat until 1933 so the Ethiopians may not receive any arms embargo from the Great Powers. But whether the Ethiopians will win or not, is another story.