I set up installments for my 2018 tax year then paid $341.00 to reduce it; but the 341.00 was applied to 2019. How do I get credit for 341.00?

You need to contact the IRS if they misapplied your payment.

The Phone number is 800-829-1040, but it can be hard to get through on that number.

If you were given any contact information while you were setting up your installment plan, try that first.