If you are monitoring an Android device, capturing incoming instant messages works on rooted devices only. You need physical access to the device to install SpyStealth.
SpyStealth runs on jailbroken/non-jailbroken iOS devices. In the former case, you need physical access for installation. In the latter case, you need no access if you have iCloud credentials, but you get fewer monitoring features. Still, the access may become necessary if iCloud backup isn't activated on the device.
Your device must be compatible to run SpyStealth software.