I am trying to create a bootable USB of Mountain lion using Sequoia 15.4.1

I don’t think there is any code. The createinstallmedia utility did not exist in installers prior to El Capitan. I doubt Apple injected it into the older installers they made available.

Is that Mac not able to boot up in internet recovery?

Picking up on @Barney-15E idea of " boot up in internet recovery? "

Have you tried booting the computer in Internet Recovery Mode

Option + Command + r - three keyboard combo

Start up your Intel-based Mac in macOS Recovery

OPs' question " I am trying to create a bootable USB of Mountain lion using Sequoia 15.4.1 "

Not possible

To perform this action will require booting from a Bootable Installer. 

The Bootable Installer can Only be performed on an Apple Computer 

This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer.

Example : Bootable Installer of Mojave would have to be done on a computer that Qualifies to run Mojave.

Notation: If the computer being used to perform this action is Too New to qualify to run the version of macOS - this computer can’t be used.

Alternative is, to gain access to a Qualifying Apple Computer from a family member, friend or associate.

It's been a while since I created a bootable installer for Mountain Lion and AFAIR I then had to purchase it while it now seems to be more readily available.

There was an older slightly more convoluted method to create a bootable installer but here is the more recent method that worked in the past. But Apple might have changed the folder structure later so this might not apply anymore. And this might not work in the newer Sequoia Disk Utility especially when that Mac does not support Mountain Lion.

Copy the following file out: Install OS X Mountain Lion.app > Contents > Shared Support > InstallESD.dmg.

Open Disk Utility and select the Flash Drive device name (not its volume or partition). Select the Erase tab on the right and then set format to Mac OS Extended (Journaled) and GUID Partition Map.

Select the Flash Drive volume (not the device), select the Restore tab at the top and then select the InstallESD.dmg. Click Restore and wait for the restoration to finish.

If that does not work, you'd better ask here or some other forum:

https://discussions.apple.com/community/macos/earlier-os

This is going to be a mess, because the necessary gear here is lacking, and these old systems are really rather limited around “modern” features like recovery. Recovery prior to macOS 10.12.something is messy.

You can build a bootable installer, but you'll have to work at it. Start by a web search for the details specific to the macOS version. Maybe this: https://www.lifewire.com/create-bootable-copies-os-x-mountain-lion-installer-2260352

The downloads of the older OS X versions can be doubly-packed too, where you’ll have to download and then perform extra steps. And some of the older versions have broken certificates, which means altering the current date, or altering the installer itself.

The results of the install can be less than useful as well, as many modern websites won’t work that far back.

Mac OS X Mountain Lion Installer - Apple Support

As for the path I’d follow, if Recovery Command-Option-R doesn’t work, I’d download macOS 10.15, and then wipe. (Among the many wrinkles here is if the Mac is missing an older firmware update that macOS 10.15 doesn’t try to install.)

The other option for newer tools and newer browsers is BSD or Linux, which will probably boot and run here. macOs 10.15 is getting pretty old, and macOS 10.13 and earlier are getting progressively more problematic.

Maybe try Target Disk Mode and install from your other Mac.

On the old Mac, hold down the T key during Startup. Then, connected the two devices with a compatible cable/adapter.

Transfer files between two Mac computers using target disk mode - Apple Support

Once you mount the old Mac, you could try to install Mountain Lion or another version up to Catalina.

However, the Mac you are using to install the OS must be able to run that OS version.