iMac bootet nicht von Usb Stick .

TransMac does not seem to work for creating bootable macOS USB installers for the last few years. Plus a few years ago Apple changed the macOS 10.11 installer which is now downloaded as a .dmg file, however, the contents of this file is just an app which is used to extract the real installer to the Applications folder. This app requires using a Qualifying Mac from 2008 to 2015. If you have access to such a Mac, then you can use these Apple instructions for creating a bootable macOS 10.11 USB installer:

How to create a bootable installer for macOS - Apple Support

A few years ago I did stumble across some command line instructions for creating a bootable macOS 10.11 USB installer, but those instructions were a bit confusing even for someone who is knowledgeable & familiar with the command line. Plus I've seen posts the comments of those instruction since then mentioning that later versions of the macOS 10.11 .dmg file had changed so those instructions had to be modified. Unfortunately I don't currently have a link to those instructions which are most likely outdated now.

FYI, even if you install macOS 10.11 on the Mac, it will be severely outdated where currently only Chromium based browsers are supported. I don't expect them to be supported for much longer. A better option may be to install Linux Mint on the Mac which would provide an up to date non-Apple OS with support for all the current popular browsers (Firefox, Google Chrome, Vivaldi, etc.) plus access to lots of free open source apps such as LibreOffice. There are even Linux versions of Zoom and Teams available. Linux would probably end up performing a lot better on this older hardware than macOS will (assuming the hard drive is still healthy and the system as 4GB of RAM although Linux can theoretically run with as little as 2GB of RAM if used carefully).

What year model iMac are you working on?

see > Identify your iMac model - Apple Support

I'm pretty sure that DiskMaker 6 can still make an El Capitan installer on a USB flash drive or external drive.

see > https://diskmakerx.com/download/

Plus you may be able to use a later version of DiskMaker X depending on the year model of the iMac and what macOS it can run.

FWIW I have High Sierra on a USB installer made with DiskMaker 7, that I occasionally use when I'm working on older 2010 and 2011 Mac's.

"Hello, as a donation for Ukrainian people, we have an iMac, with Os X El Capitan, when installing El Capitan via USB stick, it would not be recognized as boot media (created with Transmac and Os X El Capitan .dmg) all key combinationsalready tried, unfortunately unsuccessfully. How can I boot from a USB stick? By the way, Macintosh HD disk is erased but Apple disk image media is not, should I erase it too? Many thanks for the help"

Insert the bootable flash drive into a compatible port, press the start button then immediately press and hold the Option key. If the flash drive is bootable, you'll be able to select it to boot from.

To erase or not is your call. If your intention is to install the OS from the flash drive, it won't matter.