Click the hyperlink in the software What if I didn't keep track of my mileage? It says:
What if I didn't document my miles?
The IRS requires keeping a mileage log, which tracks the date, start/stop location, business purpose, and total mileage of each drive.
If you didn't track your mileage this year, consider creating a log for your records using your calendar or work schedule, vehicle service records, and/or bank statements. Keep one of these logs for the future. The IRS has a sample log on Table 5-2 in Publication 463 (page 39 of 61), or there are mileage tracker apps that can help you.
Be as exact as possible with your mileage. Because of stricter rules for recording vehicle expenses, numbers that seem like estimates may make you more likely to be audited.