You have two option to deal with it.
1. You can reply on the notice that you were not registered and not liable to pay, However It depends on multiple things. You can further go for appeal also.
2. You can avail benefit of SVLDRS - SABKA VISHWAS (LEGACY DISPUTE RESOLUTION) SCHEME, 2019 {In that case you have to register in this scheme through below link and you have to pay 30% of Tax Amount (which is mentioned on notice), Interest & Penalty would be waive off & Case/Order will be close immediately against you}, However Tomorrow is the last date for apply in this scheme.
https://www.cbic-gst.gov.in/cbec-portal-ui/?amnestyScheme