I selected to debit my bank account on 4/15. I have to pay in this year. Once my return is accepted, how many days does it normally take the IRS to process a charge?

The IRS will debit your account on 4/15/2019 or within 2-3 days following.  They're usually pretty prompt about getting their money.  

Crud. I get paid on 4/18 and I was hoping it would be after that pay cycle for me.. any suggestions?

I'm so sorry.  You may try contacting the IRS directly at 1-800-829-1040 and explaining the situation.  IF you can get an agent on the telephone, they may be able to delay the direct debit for a few days.  But you'll incur a late payment penalty (just for the few days).  
That's the only thing I can think of.