In order to find the registration code:
1. Log in to your mSpy account at my.mspyonline.com
2. Tap "Get Started" > Select "Android"
3. Keep tapping "Proceed" > "Next" until you see the "Registration Code" (the number marked in red)
The registration code needs to be entered on the target device during the installation. It cannot be received in case the device is already linked.
If you are changing the monitored device, you need to unlink the device you monitored before from your account to get the new registration code, as it changes any time you unlink like a new device. Please follow this guide first.