I am growing tomatoes in coco coir and they are now 7 weeks old. some leave are curling inward. could someone please suggest what could be the reason for this.
Its name is "tomato leaf roll' (you can Google that term) and it is a physical response of the plant to stress of some kind. It isn't necessarily harmful unless it becomes really severe but it is important to identify the cause of the stress and eliminate it when possible.
Normal garden plants respond this way when exposed to inconsistent or excess water such as after a heavy rain. But in your artificial environment there could be several other causes or contributing factors - the growing medium, the soil pH, the nutrients (excess or insufficient), insufficient light, container size, etc. etc.
You will need to evaluate and/or modify all your parameters to identify the cause or causes.