I screwed up my Bootcamp partition. I want to restore HD to single volume.

So I've been using Bootcamp for years and have never had any issues getting rid of the Windows partition. Usually, I could do it in the Bootcamp Assistant, but this time I couldn't. A dialog box said :

"The startup disk cannot be partitioned or restored to a single partition. The startup disk must be formatted as a single Mac OS Extended (Journaled) volume or already partitioned by Boot Camp Assistant for installing Windows"

This has never happened before.

So I screwed up and did some terminal code I found online and then tried deleting using Disk Utility, but it won't go away. I have still a "BC1" internal drive partition that I need to get rid of.

I tried restoring High Sierra but the "BC1" partition is still there.

How can I completely format my Mac hard drive to a single volume?

Thanks.