You should report this check as income in 2018 as you received the check in 2018. The IRS considers that the money was available to you in 2018 and it counts as income in 2018.

Per IRS Publication 538, generally individual taxpayers must adopt the cash method and a calendar year for tax purposes. Under the cash method, you include in your gross income all items of income you actually or constructively receive during the tax year. Income is constructively received when an amount is credited to your account or made available to you without restriction. You do not need to have possession of it. If you authorize someone to be your agent and receive income for you, you are considered to have received it when your agent receives it. Income is not constructively received if your control of its receipt is subject to substantial restrictions or limitations.