If you are getting an error message that says “Please enter all details” despite filling all the particulars for two tickets on 15 12 21, there could be a few reasons why this is happening. Here are some possible solutions:

1. Check if all the fields are filled correctly: Make sure that all the mandatory fields are filled in correctly, including passenger details, date of travel, train number, class of travel, and payment information.
2. Clear your browser cache: Sometimes, browser cache can cause issues with form submissions. Clear your browser cache and try again.
3. Try using a different browser: If clearing your browser cache doesn’t work, try using a different browser to fill in the details and submit the form.
4. Use incognito/private browsing mode: Sometimes, browser extensions and add-ons can interfere with form submissions. Try using the incognito/private browsing mode to fill in the details and submit the form.
5. Contact IRCTC customer care: If none of the above solutions work, you can contact IRCTC customer care for assistance. They can help you troubleshoot the issue and guide you on how to proceed with your booking.

I hope this helps you resolve the issue you are facing while booking your tickets on IRCTC.