This error message will appear when the Template Id passed by you is not found on the DLT platform.
You might be making one of the below mistakes:
1. Passing Entity Id instead of Template Id
2. Passing Template Reference Id instead of Template Id
3. Missing 1-2 digits from the Template Id
You should immediately find the correct Template ID and start passing the same.
To find the same, kindly download your approved Templates from the DLT platform - https://msg91.com/help/export-approved-templates-from-dlt-portal