How legal is this invoice?

No GST number and the seller insisted on cash payment. But my mother-in-law doesn't think there's any issue. What troubles can I face later on? Any taxation experts on r/Assam? https://preview.redd.it/qfdo22ph8ihe1.jpg?width=2922&format=pjpg&auto=webp&s=1ece889b4bb96b21de2359bd3e1af0220c426e60