Unable to run Bootcamp on Mac shows windows error code "0xc0000255"

You can either perform a PRAM Reset (hold it for at least two or three chimes if possible) in order to remove the default Startup Disk option which apparently is configured to be Windows....or you can Option Boot the system by holding down the Option key immediately after the startup chime to access the Apple bot picker menu so you can select the macOS boot volume. Then try running Bootcamp Assistant to try to remove Windows to merge the Windows partition back into the macOS boot volume. If that does not work, then it is unlikely Disk Utility will delete the partition & allow it to be merged back, but that is something you can try if Bootcamp Assistant does not work. Even if you can remerge the Windows' partition into the macOS partition, you likely will see get a failed Windows boot since the Windows bootloader is still located on another hidden ESP partition (we can deal with that if necessary).

Otherwise you will most likely need to erase the whole drive destroying all data on it so you can start over by reinstalling macOS followed by restoring from a backup.

Did you force a power off of the iMac first?

Try an SMC Reset.

Make sure to disconnect all external devices except for your wired USB keyboard which should be connected directly to the iMac.

Timing is critical for those special startup keys to work. On some Macs, holding the keys too soon will prevent the special startup keys from being seen or it may even cause the Mac to "pause" until the keys are released....I've seen both things happen, while other Macs seem fine holding the startup keys before the startup chime.

Also, pressing the special startup keys too late will cause the Mac to boot to the default startup disk, whatever that may be.

Unless your speakers are bad, you should be hearing a startup chime if the PRAM Reset is successful since it resets the volume settings to default.

If none of this helps, then I think you will need to take your Mac to Apple or an Apple Authorized Service Provider for assistance.

Force a power off of the computer. If it is an iMac as listed in the footer/signature, then just unplug it for a moment.

When you power it back on, make sure to either perform a PRAM Reset or hold the Option key immediately after hearing the startup chime. The PRAM Reset will clear out the PRAM/NVRAM which holds the default boot volume so the Mac will search out bootable volumes and should default to the internal macOS boot volume, theoretically.

At the very least an Option boot (holding the Option key immediately after hearing the startup chime) should give you Option Boot screen (aka Apple boot picker menu....even if there are no boot options listed).

If you are using a wireless keyboard, then try using a wired keyboard since the wireless keyboard may not go ready soon enough to allow the special startup keys.

Edit: I need more details on what actually happens when you try any of the suggestions. Saying it does not work tells me nothing...I need to know what actually happens when you try the suggestion. Remember I cannot see anything and must rely on your observations & updates to know what happens exactly. I need information in order to know what is happening even if the suggestion does not work.

If you don't hear a chime while performing the PRAM Reset, either it was not performed correctly (timing is everything) or you have a hardware issue of some sort. Make sure to use a wired USB keyboard since a wireless keyboard may not go ready soon enough to trigger special key sequences.

You need to configure the default startup volume in the Startup Disk System Preferences/Settings.

Unfortunately to properly remove Windows you needed to use Bootcamp Assistant to remove Windows & restore its partitioned area into the macOS partition/container. Most people are unsuccessful using Disk Utility to merge the deleted partition free space into the macOS partition/container. You may have more luck using the command line utility "diskutil" to merge them, but I don't have the time or spare drive to experiment. I do know that a few years ago I assisted someone in that endeavor & they were able to take my slightly incorrect command & made it work. Unfortunately I don't recall the exact options used with "diskutil". You definitely need to have a good backup of macOS first since you could accidentally wipe out the macOS partition.

To get rid of the Windows bootloader you will need to mount the hidden ESP partition containing the bootloader and delete the "windows" folder from it. Assuming a single internal boot drive (not a Fusion Drive), the Terminal command needed to mount the hidden ESP partition is:

diskutil  mount  disk0s1

Use the Finder to view this volume which will be labeled "EFI" and mounted at "/Volumes/EFI". Delete the "windows" folder, then unmount the "EFI" volume. You should no longer have the iMac trying to boot into Windows even if the default startup disk is not configured.