Nice to see you in the thread, @Lori0830.

Let me guide you in paying a bill in QuickBooks Desktop (QBDT) so you can enter ACH in the Check No. column.

Here's how:

1. Go to the Vendors menu at the top. 
2. Click Pay Bills.
3. Select the Bills to be paid.
4. In the Payment section, enter the date.
5. Choose Check as the Method.
6. Toggle the Assign check number radio button and select Account.
7. Click Pay Selected Bills.
8. In the How do you want to assign check numbers question, select Let me assign the check numbers below.
9. Enter ACH in the Check No. column.
10. Click OK.

For your visual reference, you can watch the video found in this article: Pay Bills in QuickBooks Desktop.

If the issue persists, let's run the Verify Rebuild Data tool that helps identify and repair data issues within your company file. But before doing so, let's ensure that your QuickBooks Desktop is updated to the latest release.

Please let me know how things go on your end. I want to ensure this gets resolved for you. Take care.