I don’t know the beginning and ending odometer numbers. How do I get around this?

use your best estimate.  realize that with no documentation, if you are asked if the documentation is written you'll have to answer no. 

I don't think TurboTax should be requiring and odometer readings.  More over I think it should be removed. Lines 6a and 6b should be sufficient for standard milage rate 

I understand this is "an option" calculating miles, but the TurboTax "Car and Truck Expenses Worksheet" is actually not a IRS worksheet and does NOT qualify for adequate evidence as defined by the IRS.

Please note odometer readings don't even qualify in generating adequate evidence of a business expense.

https://www.irs.gov/publications/p463#en_US_2022_publink100034069

Adequate evidence.

Documentary evidence will ordinarily be considered adequate if it shows the amount, date, place, and essential character of the expense.

Also see https://www.irs.gov/pub/irs-news/fs-06-26.pdf