Depreciation is required, so you missed out on previous deductions.

Go to a tax professional to fix this.

You can enter the asset in TurboTax and the program will calculate the prior and current year depreciation and list them on the entry screens. You can note the amount and then delete the asset entry if you don't need it in the current year.

You can also Amend your tax returns going back three years and get credit for the previous depreciation.  You must complete Tax Year 2017 by April 15th of this year.

Be sure that dates and amounts match with what you posted to 2020.

@gwoolery12

If you have not taken any depreciation  at all then you should not amend anything ... instead you file a form 3115 with the 2020 return to fix this error.   The form 3115 is not a DIY project and the program will not do it for you so PLEASE seek a local tax professional for the 2020 return and get this fixed correctly.  

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@JohnB5677 wrote:

You can also Amend your tax returns going back three years and get credit for the previous depreciation. 

@gwoolery12

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No you can't amend (assuming you incorrectly did not take depreciation for 2 or more consecutive years.

As Critter pointed out, you need to file Form 3115, which is why I said to go to a tax professional to fix things.