You have to complete a Form3115. Form 3115, Change in Accounting Method, is used to correct most other depreciation errors, including the omission of depreciation. If you forget to take depreciation on an asset, the IRS treats this as the adoption of an incorrect method of accounting, which may only be corrected by filing Form 3115. You only need to complete Schedule E.
See About Form 3115, Application for Change in Accounting Method
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