Investing is great for retirement funds, over the 30 years or so it's great for returns. It's terrible to invest principle you need in scope of months or 1-2 years even.

If you have 20k and a 20k loan, putting that money in the stock market is stupid if you need that money to pay the loan. If you have 20k, a 20k loan, and income to cover loan payments in addition to living, it may be beneficial to invest that 20k, but it's still relatively high risk.