Looking at the documentation we are seeing that the crossbrowsertesting tool support various Operating systems and thus corresponding browsers as mentioned in the below link
https://crossbrow...
Correct. That's exactly what's happening. So, because of that, you can't run your automated tests against Ubuntu or MacOS because you can't run TestExecute on those. If you're familiar with doing Mobile testing with TestComplete, you know that testing on an actual device does not consist of running TestExecute or TestComplete on that device but connecting it to TestComplete. This is not possible through the cloud environment, hence why Android and iOS cannot be used with TestComplete integration currently.
basically... it's a limitation of the technology... you can't run a Windows application (TestExecute) on a non-Windows operating system...