What allows the U.S. Congress to collect individual income taxes?
The 16th amendment permits the United States Congress to levy taxes on the individual incomes without dividing the revenue to the states. The amendment was passed in 1909 and launched in 1913.
Before the 16th Amendment, the Income Tax Act of 1894 was a valiant attempt to apply a 2% income tax to individuals earning over $4,000 per year. To be clear, $4,000 amounts to $108,000 in 2017's dollars. The act was sent to Supreme Court and deemed unconstitutional. The income tax was removed after the Civil war because the government didn't need to finance the war any longer. The act was the start of what became the 16th amendment (which if a tax or bill or law is deemed unconstitutional, it needs to be in the constitution for it to work in the future).