Therefore any allocation of funds from your income that doesn't fall into basic or must do expenses enters into your discretionary funds.
Long story short, "Invest money you can afford to lose"... It's like the first and basic rule of investing, and gambling. "Safe investments" are not so profitable in the short run, and investing some small amounts of money will not bring "meaningful profit" in the long run. So most people like risky investments, and when it's risky, there's always a chance of losing funds.
If you want to invest in a long term profitable asset and you don't have a discretionary fund you can do the following. Cutdown on your basic expenses if you can, get a better paying job, get a side hustle, learn profitable skills, do what you can to earn more than your basic expenses.
Eh, some things are so easy to say, but when it comes to making it real, there can be a lot of obstacles... I guess we are all fighting the best way we know how. We fall & rise constantly... there's always something new to learn.