Hi there! I understand you're seeing Error Code 173 when trying to purchase a ticket on the MTA TrainTime app on your Mac running OS X 10.15. Let's work together to get this sorted out.

To help me assist you better, could you tell me:

- Are you using the MTA TrainTime mobile app or the website on your Mac browser?
- Have you tried purchasing with a different payment method or credit card?
- Have you already tried removing and re-adding your payment info in the app?
- Is your app updated to the latest version available?

Meanwhile, here are some steps that commonly fix Error Code 173 related to ticket purchases:

1. Verify Payment Details: Double-check that your credit card info and billing address match exactly with your bank records. Mismatches often cause payment errors.

2. Remove and Re-Add Payment Method:
- Open the TrainTime app.
- Go to "More" at the bottom toolbar.
- Select your payment options, delete the current card.
- Re-enter your card info carefully.

3. Update the App: Ensure the TrainTime app is updated from the Mac App Store to avoid bugs that could cause this error.

4. Log Out and Back In: Sometimes signing out of your account and signing back in refreshes your session and resolves issues.

5. Check Network Connection: A stable internet connection is essential for purchases. Try switching from Wi-Fi to wired or another network if possible.

If you've done all this and still get the error, the problem could be linked to something like an invalid app receipt or a backend payment processing issue specific to your account or card.

Let me know which of these steps you've tried and any other details, and I'll guide you from there. If needed, we can also escalate this to MTA TrainTime support to check your account status.

Are you satisfied with the solution I provided so far?