How to contact the IRS regarding an error on Stimulus payment?

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See this IRS website for information - https://www.irs.gov/coronavirus/economic-impact-payment-information-center

A29. If you did not receive the full amount to which you believe you are entitled, you will be able to claim the additional amount when you file your 2020 tax return.  This is particularly important for individuals who may be entitled to the additional $500 per qualifying child dependent payments.