* Some programs require the GRE: If your I-20 is for a program that explicitly *requires* a GRE score as part of the admission process, then a lack of a GRE score will be a significant problem and likely prevent you from getting a visa. They wouldn't have issued you the I-20 without it if it's a requirement.
* Some programs don't require the GRE: Many programs, especially at the master's level, are now GRE-optional or have waived the requirement. If your program falls into this category, the GRE score is less critical, although you should still be prepared to discuss your academic qualifications and suitability for the program. The consular officer will focus on your overall profile, including your academic record, letters of recommendation, statement of purpose, and financial resources.
* Demonstrating readiness is key: Even if the GRE isn't required, having a strong academic record demonstrating your capability to succeed in your chosen program is crucial for a successful visa interview.
In short: Check your admission letter and any communication from the university regarding GRE requirements. If it was required and you were admitted without submitting it, there might have been an exceptional circumstance. However, *you should contact the university's admissions office immediately* to clarify the situation and ensure there's no misunderstanding. Ignoring a requirement could jeopardize your visa application. If the GRE wasn't required, focus on other aspects of your application to demonstrate your eligibility.