Apple watch SE series 2 not connecting to airtel postpaid sim

Thanks for clearing the confusion. Your postpaid SIM should support cellular service for any currently marketed Apple Watch.

Your comment is <interesting> because it appears Apple Watch SE is marketed in India, so there is no reason Airtel would not support it. I think they need to be challenged, and if no success be prepared to escalate the problem to Airtel Supervisors.

Please see Apple Watch SE - Apple (IN) for the evidence. That page tags footnote (5) for cellular capability with the SE, and in turn the footnote references Set up cellular on Apple Watch - Apple Support (IN)

Apple's page Apple Watch - Carriers - Apple (IN). shows clearly that Apple Watch SE is supported in India, with models:

• Model A2726 (40mm)
• Model A2727 (44mm)

I suggest you check the model number of your Apple Watch.

As far as I know these are the current worldwide / international (non-USA) version of Apple Watch SE which you describe as Watch SE 2nd generation. The original Apple Watch SE has been discontinued for some time, and there is only one version (second generation) available now. However, note that the USA/North America version uses different radio frequencies and if you have purchased on overseas it will not be supported outside its home region.

There is confusion in your post the subject line says "postpaid" and the message body says "prepaid". The difference could be important if you are trying to activate the cellular version of the Apple Watch. In most cases a cellular service for the watch is not available with a prepaid SIM for the host iPhone.

if you are simply trying to activate your Apple Watch SE using the Bluetooth function to pair with your iPhone, it should work for any current iPhone. See the notes and just follow the steps in in Apple's support document Set up your Apple Watch - Apple Support