If box 2b is marked, the insurance company is saying that they do not know the taxable amount nor the amount of premiums paid. Simplest will be to just enter a zero in box 2a to tell TurboTax that the taxable amount is zero.
The IRS might end up questioning your determination that the taxable amount is zero. If they do, you'll need to provide the IRS with documentation to support your assertion that the premiums paid totaled $23,000.
And is box 2b Taxable amount not determined marked?
Any amount in box 5?
If box 2b Taxable amount not determined is not marked and box 2a is zero or blank, the Form 1099-R is indicating that the distribution is nontaxable because the amount distributed is less than the premiums paid. The Form 1099-R must still be reported even though usually an insurance company wouldn't even issue a Form 1099-R under these circumstances.
If box 2b is marked, the insurance company is saying that they do not know the taxable amount nor the amount of premiums paid. Simplest will be to just enter a zero in box 2a to tell TurboTax that the taxable amount is zero.
The IRS might end up questioning your determination that the taxable amount is zero. If they do, you'll need to provide the IRS with documentation to support your assertion that the premiums paid totaled $23,000.