I dit not issue any payroll in April. QB generated a tax due

The reason you're seeing this is that 941 taxes cover three-month periods. The April tax you see is likely for January to March. April's pay is part of the next period (April to June), with taxes due in July. So, the tax shown is probably from earlier months, not in April. 

To check where the 941 tax amount comes from, open the 941 form in QuickBooks and find the date on the form. Then, compare it with the actual paychecks to see if the dates and amounts match up. See the examples added to my screenshots below.

You can visit this article for more information to learn more and understand how your payroll taxes are being calculated and reported correctly: How QuickBooks populates Form 941.
 

Additionally, you can also look at or print your payroll tax payments and forms in QuickBooks Online. It helps you see all your payroll tax information in one place.
 

If you have any other questions or concerns related to payroll, leave a reply below. We're here to assist you.