Blynk.syncVirtual(vPin) is used to force the server to tell you the current value of the widget attached to vPin when you don’t currently know it. This is usually when your device first boots up, or when connection with the Blynk server is lost then re-established.
Calling Blynk.syncVirtual(vPin) causes the BLYNK_WRITE(vPin)` callback to be triggered.
The BLYNK_CONNECTED callback is triggered whenever the connection to the Blynk server is first established or re-established. This is why Blynk.syncVirtual(vPin) is placed in the BLYNK_CONNECTED callback.
There is no reason for you to write data to the server then immediately get the server to tell you what that data was. You already know that information and this approach would potentially create an endless loop.
I don’t want to share code because of some reasons. The code is same like this as you said there are some libraries missing, in my actual I have include all necessary libraries
Well, unless you’re prepared to share the code which makes the sketch useable, and which populates the StaticJsonDocument with values then I don’t think you’ll get much assistance from Blynk community members.