I am unable to enable billing . it’s asking for increase in the billing quota . Please help me out as i got stucked here !!

Perhaps you’ve exceeded the "Billing linkage quota" with the free tier account. Basically when using the free tier of GCP, you are only allowed to enable Billing (linking projects to billing accounts) on a limited number of projects. Mine case it’s about 5 projects, means that I cannot have 6 projects with Billing enabled.

So you might want to disable Billing on some existing projects that you don’t use (unlink Billing Account from those projects), then you can enable Billing on new projects.