I want to receive text from arduino , but where to place that text , on a label or call a notifier?
What control you use to capture the text from your arduino is your design issue Younus.
A Label is probably a great solution.
A notifier might be a good idea. Remember, the Notifier displays only 'short' or 'long' duration and might cause you timing issues.
What you use depends on how frequently the text 'arrives' and how you want to use the control in your app and how long or extensive the text might be. App Inventor is flexible. Notifiers are not really for large volumes of text but Labels handle that just fine. Try using a Label; then see what happens if you use the Notifier.
If there are no issues, use what looks good and provides the information you want. 
I would first save the incoming message in a global variable, because you will probably need to refer to its contents a few times in different if/then tests to respond appropriately to the incoming message.
For display purposes, if you are still testing, I prefer using logs, in a label or a list picker:
https://groups.google.com/d/msg/mitappinventortest/II_RlElGddU/J0q_E-2rAwAJ
Hi, I created a playlist on my channel about Arduino AppInventor 2 via Bluetooth, from the connection to sending and receiving digital and analog data (including texts) and with a small HMI. Maybe this can help.