Purchase from e platform without gstin

Mr. Pankaj is correct, don't contact Amazon, they cannot help now. Instead, contact the actual dealer/seller directly and request them to add your GSTIN in their sales record and issue you a revised bill. MAKE SURE you are able to see that invoice in your GSTR-2A upon the seller's monthly/quarterly GSTR-1 filing, because many times sellers fail to report subsequently added buyer GSTINs in their GSTR-1 and therefore wrongly show them in B2C sales, in that case, you will have to follow-up with the seller and ask them to reduce(amend) their B2C sales figures in next return and show your invoice in B2B.

Going forward,  you should either convert your existing Amazon personal account to "Amazon Business" or create a new "Amazon Business" account wherein, at the time of sign-up/registration you will be REQUIRED to provide your GSTIN, which Amazon automatically shares with the actual seller for GST billing, upon placing an order.

NOTE: To ensure ITC is available to you, ALWAYS login with your Amazon Business account if you want to place orders for your business.