There are already 3 good answers to this question, but I'll try to complement it a bit further with the issues that make Economists disagree.
What is the purpose of economics? Does it have predictive power?
Economics as a social science is grounded in the positivism paradigm, best described according to Gunter (2000):
The "overriding objective" of the positivism paradigm is to "prove or disprove hypotheses and ultimately to establish universal laws of behaviour through the use of numerically defined and quantifiable measures analogous to those used by the natural sciences"
The point of economics is agreed by most to be finding the universal laws of economic behaviors.
Given that most real-life situations are very complex, there are often competing views and theories on what the outcome will be. This is rooted in a series of issues:
Logic and Math over empirical proof
Several models, particularly the classics, have been created from mathematical and logical thinking, before the availability of data. These theories and models are logically and mathematically very sound. The issue here is the applicability to solve real-life problems, which becomes a topic for disagreement between economists.
One of the examples is Ricardo's theory of comparative advantage, which is used as a basis for major consensus in the economics profession, and a key driver of the free trade movement, was prepared before the existence of large scale trade data. Nowadays, we have several empirical studies that disprove the applicability of comparative advantage theories for international development: African countries with free trade have not developed near as fast as Asian countries with import tariffs and export subsidies (Piketty 2014, Galbraith 2008) The theory of comparative advantage is so broadly accepted that one could compare it to a "law" in economics, but other sciences would reject any law which is, at least once, proven to be wrong.
Piketty describes this issue very elegantly:
To put it bluntly, the discipline of economics has yet to get over its childish passion for mathematics and for purely theoretical and often highly ideological speculation, at the expense of historical research and collaboration with the other social sciences. Economists are all too often preoccupied with petty mathematical problems of interest only to themselves.
This obsession with mathematics is an easy way of acquiring the appearance of scientificity without having to answer the far more complex questions posed by the world we live in.
Problem of Aggregation
Conclusions taken for one single individual cannot always be translated to aggregate terms (multiple individuals). This means that microeconomic results are not necessarily transferable to macro terms, and this can create disagreement among economists. Peston (1959) makes a good overview of the problem in the first page, or check the wikipedia page.
Economics research is highly vulnerable to bias. Even if methodologically the studies can be well grounded in logic and data, the point of view of the researcher can be influenced by one's own ideology or by political influence.
A recent study on 159 economic literatures found "half of the research areas have nearly 90% of their results under‐powered" and "nearly 80% of the reported effects in these empirical economics literatures are exaggerated"(Ioannidis, Stanley, Doucouliagos 2017)
Henry Farrell wrote an interesting model on the disagreement between economists based on things such as political influence and level of consensus.
More like Pharma, less like Physics
Although the predictive power is often expected to be as advanced as Physics, Economics is a relatively recent science compared to most natural sciences. I like to think of it closer to Pharmaceutical studies than physics: there is a lot of experimentation, and we slowly learn what effects certain things have in the system, but we often only realize the secondary effects much later. Pharma is a subsection of Chemistry, and in the same way, I see Economics as a subsection of Sociology.
If not, what's the point of economics (it might as well be astrology)?Congrats! You understood economics perfectly well. ;) $\endgroup$