Are you entering a new asset or reporting prior depreciation? TurboTax will calculate AMT Special Depreciation based on your regular special depreciation deduction. At this time, you may claim up to 80% of the cost of your asset for Special Depreciation.
If you are reporting prior depreciation, this amount can be found on the Asset Entry Worksheet Line 12, from last year, or on your AMT depreciation report that should be included with your tax forms (if you used TurboTax or any other preparation software).
Note that the IRS suspended use of Form 2106 for most employees, with a few exceptions:
"Use Form 2106 if you were an Armed Forces reservist, a qualified performing artist, a fee-basis state or local government official, or an employee with impairment-related work expenses. Employees who do not fit into one of the listed categories may not use the Form 2106 due to the suspension of miscellaneous itemized deductions subject to the 2% floor under section 67(a). Section 67(g) suspends miscellaneous itemized deductions for tax years beginning after December 31, 2017, and before January 1, 2026."