To resolve the issue indicated by the error message "Could not load merchant details needed for purchase" in the MileagePlus X app, follow these step-by-step instructions:

1. Check Your Internet Connection: Ensure that your device is connected to a stable internet connection. A weak or intermittent connection can lead to issues when accessing merchant details.

2. Update the MileagePlus X App: Make sure you are using the latest version of the MileagePlus X app. Go to the App Store, search for MileagePlus X, and check for any available updates. Update the app if necessary.

3. Clear App Cache: Sometimes clearing the app cache may help resolve loading issues. Although not explicitly available on iOS, restarting the app can help refresh its state. Close the app completely and reopen it.

4. Log In Again: Log out of your MileagePlus account in the app and log back in. This can refresh your session and resolve any temporary glitches.

5. Try the Website: If the app continues to show the error message, you might bypass it by completing your purchases directly through the MileagePlus website. Navigate to giftcards.mileageplus.com, log in, and attempt your transaction there, as users have reported success with this method - learn more - learn more.

6. Check for System Status: Confirm there are no ongoing outages affecting the MileagePlus services. You can check the United Airlines service status page or follow related updates on social media platforms.

7. Adjust Browser Settings: If you're using the website, make sure your browser settings are configured to allow cookies. Blocking cookies can interfere with the loading of necessary details - learn more.

8. Contact Customer Support: If none of the above solutions work, reach out to MileagePlus support. Provide them with relevant information about the transaction, including the error message, and they can assist further - learn more.

Following these steps should help rectify the issue and enable you to complete your transaction using the MileagePlus X app or website.