If you are a cash basis taxpayer, you generally would not report any income until you have received it.
What they did with regard to not sending the income to you until 2021, but reporting it as 2020 income is not necessarily correct. However, it may be unlikely that you will be able to get the situation corrected by the gallery.
Since the gallery has indicated on the Form 1099-NEC that they paid the income in 2020, the IRS will expect to see it included on your tax return for 2020. If you do not include it as income, you should expect a letter from the IRS proposing an adjustment to your return. At that point, you would have to prove that you did not receive the income in question until 2021 and that you had no other means to get the payment until 2021. This could prove difficult especially if the delay was due to something like the postal service.
If you determine due to the circumstances that you should report the income in 2021, but your 2020 1099-NEC includes the income, then report the full amount of income on your 2020 tax return along with an expense adjustment to account for the amount that will be reported on your 2021 return. Be sure to keep good notes for your 2021 return since you will not have a matching 2021 Form 1099-NEC reporting that income. You will have to enter it as 'cash' on your 2021 return.
Take a look at the following TurboTax article for further discussion of the cash accounting method:
What is the cash method of accounting?
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