Restaurants cannot claim Input Tax Credit ( ITC ) on their Inward Supplies ( Purchases ).
As per Rate Notification No. 46/2017 - Central Tax ( Rate ), dated 14th November 2017, Standalone Restaurants ( i.e. not within a Hotel, Guest House, Inn, etc ) are required to charge 5% GST from their customers and they will not be entitled to claim Input Tax Credit ( ITC ). This notification came into effect from 15th November 2017.
Moreover it was also clarified vide Notification No. 20/2019 - Central Tax ( Rate ), dated 30th September 2019, that Supply of Restaurant Services "other than at Specified Premises" would attract GST @ 5% without the benefit of ITC. This Notification came into effect from 01st October 2019.
Therefore you cannot claim ITC on Restaurant Services unless you are providing Restaurant Services within a "SPECIFIED PREMISES" ( like Hotels having declared tariff of more than Rs. 7,500 per unit per day ).
NOTE : "SPECIFIED PREMISES" means premises providing hotel accommodation services having declared tariff of any unit of accommodation above Seven thousand Five hundred rupees per unit per day or equivalent.