It depends on what you mean by closed and how your LLC elected to be taxed — as an S-corporation or partnership.
If by closed, you mean you terminated your LLC in 2020 and it no longer exists, then you do not have to file a tax return.
If you closed your business but still have an LLC then you would have to file an S-Corp return. Partnerships must file, unless you neither receive income nor incur any expenditures treated as deductions or credits for federal income tax purposes.