A personal loss is no longer deductible ... however if this was a business asset then you will take it off your asset list. If it was not fully depreciated already you will sell it off for the amount of insurance you received for the loss if any. If you had expensed the cost in a prior year then there is nothing left to take.
It’s not a deductible theft loss on schedule A. (Personal itemized deductions.) Theft losses were disallowed by tax reform.
If you are self employed, and the bike is an ordinary and necessary expense of your profession, then you could have listed as a business asset when you bought it, and taken depreciation. What happens next depends on whether it is fully depreciated or not, and whether it was covered by insurance. If you depreciated the bike, your insurance reimbursement might even be taxable income.
If you never listed as a business asset, you would have to go back and amend your tax return for the year you placed it in service to depreciate (deduct) all or part of the cost as a business expense. Then what happens on your 2019 return again depends on insurance and depreciation.
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