Should I take a special depreciation deduction for a new septic system if I plan to sell my rental property this year.?I

It's your personal choice. Please see the information below:

• If you take the special depreciation deduction in 2021, you may pay less income taxes, in 2021.  
• However, since you're going to sell your rental property in 2022, you may have to recapture the special depreciation you took in 2021 and all the previous years of depreciation that you've taken and pay taxes on it in 2022, which will result in having to pay more income taxes in 2022.

When you sell a rental property, in addition to capital gains, you may have Depreciation Recapture. If you dispose of depreciable or amortizable property at a gain, you may have to treat all or part of the gain (even if otherwise nontaxable) as ordinary income. Per IRS Publication 544 Chapter 3 Depreciation Recapture : page 27:

• You depreciated the building and other assets in years past. 
• You received the benefit of a depreciation deduction that offset ordinary income tax rates (a potential Federal tax savings of up to 39.6%, based on current rates).
• The government is not going to grant the most favorable capital gains rates on the portion of the gain relating to these prior depreciation deductions.