How do I fix this on myownfreehost?

You’d be better asking myownfreehost for help with that. Most people here don’t have a myownfreehost account. That said the error message is quite self explanatory. The domain is in use somewhere on the IFastNet network. It looks like you may be using it on your infinity free account. You’ll need to remove it before you can add it to your myownfreehost account