Gstn within state but supply outside the state(gst allocation in Gstr_1)

Change the place of supply in GSTR1 from Kerala to Tamil Naidu..issue will get resolved.

Sir,it's not allowing to change place of supply.based on gstn its automatically taking kerala state.

This is the case of Bill to ship to , Section 10(1)(b) of the IGST act 2017 , in this case place of Supply will be , Principal place of Business , That is Mr A Kerala , So in my opinion you have to charge CGST and SGST to Mr A and portal is showing correctly .
Goods were sent to Tamil Nadu on the direction of Mr A , so bill to Mr A and ship to Tamil Nadu site , So it's bill to ship to case . I hope Tamil Nadu site is not registered under GST.
This is my opinion , please take others opinion also .

Yes correct Mr. Prasad , Actual Recepient is Mr. A , even though the material has been delivered to tamilnadu on instructions of Mr. A , it's deemed that it being received by actual Recepient 'A' , So on bill also you need to charge CGST/SGST as both supplier & Recepient are in same state.