It seems that you may have forgotten to download your migration certificate despite having the transaction ID and acknowledgement number. When you attempt to download it, you are encountering an error message stating that your acknowledgement number is invalid. Here's what you can do next:

1. Reach out to the university where you applied for the migration certificate. Kindly explain that you previously applied and have the transaction ID, but are currently unable to download the certificate.

2. Provide the university with your transaction ID and the date of your application, and kindly request them to check the status of your application and assist you in obtaining the migration certificate.

3. The university staff will then be able to look up your application using the transaction ID. They will either provide you with the migration certificate or guide you through the next steps.

4. In case of any issues or delays, politely follow up with the university until you receive the migration certificate. Communicate the urgency of the certificate for your further studies.

5. Remember not to delay in reaching out to the university, as migration certificates are time-sensitive documents.

The key is to communicate with the university using the details you have. With some follow-up, they will surely help resolve the issue and assist you in obtaining the migration certificate you have already applied for. Good luck!