You can follow below steps to install Onespy on your phone. Please note below mentioned steps are for a generic Android phone, you phone may have similar options with different names depending on your device vendor.
Go to Settings > Google > Security > Google Play Protect.
Disable Improve harmful app detection and Scan device for security threats.
Then, go to Settings > Lock Screen & Security > Unknown Sources should be enabled.
Go to Chrome browser.
Click on 3 dots (menu button) in the top of the right side.
Click on New Incognito Tab & type d.onespy.com in the URL bar at the top.
Enter the email address you used while purchasing and click to Download button as per your Android version.
Once download completes, click on the file to Open it.
Then Next > Next > Install.
Click on Open or Launch.
Put your email address & password (received after payment) then click on Register.
Select a license to associate with this device.
Clear download history.
Go to Settings > Accessibility. (Important! For WhatsApp on unrooted phones only)
Scroll down, look for Photos Viewer and turn it On. (Important! For WhatsApp on unrooted phones only)