I don't fully support the sentence "If you don't have reserve fund you shouldn't invest". Because if you stick to your goals and if you have full effort you can succeed in your investment without any funds. In this case you may have to reduce your quality of life. You will have to reduce your monthly budget (under adverse circumstances). Sometimes you may need to take out a loan, which you have to pay off by reducing your monthly budget and saving extra money. The essence of this strategy is to lower your standard of living.
Many will not like this strategy. Many continue to invest in this strategy and succeed. If your goal is firm, you can invest without reserve funds.
What you say is not easy to manage and sometimes it is impossible to do so. I think that a person without reserve funds should not take out loans to invest, that would be counterproductive and could generate more debt, and I also do not think that the monthly budget should be lowered, a person with dependent children, for example, could not do that, and neither do those who pay rent or mortgage.
I still think that before investing the most productive and safe thing is to create a reserve fund for what may happen, instead of asking for loans or lowering the monthly budget. Otherwise, the pressure that person would feel would not allow them to make smarter decisions.