Economists tend to disagree on two main types of issues. First, they disagree on issues of normative economics. Second, they disagree at times about what variables have what effects even in questions of positive economics.
Economists often differ on issues of normative economics. Normative economics has to do with opinions. It has to do with what the world should be like or how the economy should be run. So, for example, a question of normative economics is whether a flat tax or a progressive tax is fairer. There is no way to objectively determine what is fair. Economists cannot do studies to determine what is fair. It is solely a matter of opinion and economists will often disagree on such matters.
However, economists can and do differ about issues of positive economics as well. This is largely because of the complexity of a country’s economy and the difficulty of making predictions. For example, economists differ as to how much taxes should be cut or raised and how much government spending should be cut or raised in order to create stable economic growth. This is not supposed to be an opinion. It is not about fairness, it is about what causes GDP to rise. However, it is very difficult to know the exact impact in all cases of a tax increase or a spending cut. This is because there are too many variables, too many things that can affect the economy. Therefore, economists disagree. Some think that tax hikes will hurt the economy while others say they will not.
Thus, economists disagree over issues of normative economics, but they also disagree on positive economics because it is hard to trace cause and effect precisely in economics.