Probem removing Windows from my iMac

Model Name: iMac

  Model Identifier: iMac17,1

  Processor Name: Intel Core i7

  Processor Speed: 4 GHz

  Number of Processors: 1

  Total Number of Cores: 4

  L2 Cache (per Core): 256 KB

  L3 Cache: 8 MB

  Memory: 32 GB

  Boot ROM Version: 161.0.0.0.0

  SMC Version (system): 2.34f2

 System Version: macOS 10.14.2 (18C54)

  Kernel Version: Darwin 18.2.0

  Boot Volume: Macintosh HD

  Boot Mode: Normal

  Computer Name: John’s iMac

  Secure Virtual Memory: Enabled

  System Integrity Protection: Enable

From Boot Camp Assistant User Guide

for macOS Mojave

Remove Windows from your Mac using Boot Camp

You must use Boot Camp Assistant to remove Windows, or a partition that was created with Boot Camp Assistant, from your Mac.

WARNING: Do not use any other utilities to remove Windows or a partition that was created with Boot Camp.

1. Start up your Mac in macOS.
2. Back up all important data stored on your Windows partition before you remove Windows.
3. WARNING: When Windows is removed, the Windows partition—as well as all data stored in the partition—will be erased permanently.
4. 
   
5. Quit all open apps and log out any other users.
6. Open Boot Camp Assistant
7. , then click Continue.
8. If the Select Tasks step appears, select “Remove Windows 7 or later version” or “Remove Windows 10 or later version,” then click Continue.
9. Do one of the following:

• If your Mac has a single internal disk, click Restore.
  • If your Mac has multiple internal disks, select the Windows disk, select “Restore disk to a single macOS partition,” then click Continue.

message from Boot Camp Assistant:

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.

What can I do to remove Windows 10?