Search this board for connection code.
After that is fixed, fix your Delimiter handling.
Be sure to use println() at the end of each message to send from the sending device, to signal end of message.
Only use print() in the middle of a message.
Be sure not to println() in the middle of a message, or you will break it into two short messages and mess up the item count after you split the message in AI2.
Do not rely on timing for this, which is unreliable.
In the AI2 Designer, set the Delimiter attribute of the BlueTooth Client component to 10 to recognize the End of Line character.
Also, return data is not immediately available after sending a request,
you have to start a Clock Timer repeating and watch for its arrival in the Clock Timer event. The repeat rate of the Clock Timer should be faster than the transmission rate in the sending device, to not flood the AI2 buffers.
In your Clock Timer, you should check
Is the BlueTooth Client still Connected?
Is Bytes Available > 0?
IF Bytes Available > 0 THEN
set message var to BT.ReceiveText(-1)
This takes advantage of a special case in the ReceiveText block:
ReceiveText(numberOfBytes)
Receive text from the connected Bluetooth device. If numberOfBytes is less than 0, read until a delimiter byte value is received.
If you are sending multiple data values per message separated by | or comma, have your message split into a local or global variable for inspection before trying to select list items from it. Test if (length of list(split list result) >= expected list length) before doing any select list item operations, to avoid taking a long walk on a short pier. This bulletproofing is necessary in case your sending device sneaks in some commentary messages with the data values.
Some people send temperature and humidity in separate messages with distinctive prefixes like "t:" (for temperature) and "h:" (for humidity).
(That's YAML format.)
The AI2 Charts component can recognize these and graph them. See Bluetooth Client Polling Rate - #12 by ABG
To receive YAML format messages, test if the incoming message contains ':' . If true, split it at ':' into a list variable, and find the prefix in item 1 and the value in item 2.
when i connect the app via AI Companion it works pretty well and it connects with HC-05, and controll relay module via arduino uno, my issue is after building the app, and install on my android phone, after launching the app, it says ERROR 503 mac address not valid, and says cant connect..........
To debug, display the mac address in a label before trying to connect. .
Also as test disable all blocks in the Screen.Initialize event... is your error already there?
Is this an empty string?
Taifun
First, look here: FAQ Section: BlueTooth Starter Guides and fix your code accordingly.
Then, show more of your blocks and tell more precisely when the error occurs.
its empty, what should i put there, mac address for my HC-05 Module?
If you know the mac address and you do not want to connect to other mac addresses you can set it in the valueIfTagNotThere socket to connect to that address by default
Which Android version are you using for your test? For newer versions you also have to ask for permissions before being able to connect...
Taifun
let me use the mac address and see if that works, im using Android 10, which is working pretty well in AI2, but fails on its own.
Should it be like this
Well, are you sure you know what you are doing?
Do you want to automatically connect to that device or not?
If yes, then there is no need for the if statement
Taifun
sorry actually im learning 
though i consider myself a fast learner, so i will get things really quick.......
Fine. Learn about lists, FAQ Section: Lists
and save 80% of your blocks. Also, why are you adding addresses and names to a list, when it is already a list?
Read the entire thread:
thanks