AppSync Unified is supposed to be installed after jailbreaking devices because AppSync Unified is a tweak that allow people to install apps IPA packages. If you don’t install AppSync Unified, you can’t install app IPA files successfully.
How to Install AppSync Unified
Step 1: Add AppSync Unified source.
Tap Cydia > Source > Edit > Add (on the left upper corner)
Step 2: Enter cydia.angelxwind.net
Step 3：Wait a while and tap "Return to Cydia".
Step 4: Install AppSync Unified.
Tap Karen > All Packages > AppSync Unified > Install > Confirm
Step 5: Wait a while and tap "Restart Springboard".
Step 6: Check it.
Tap Cydia > Installed to check whether it is installed successfully.