Hello, I hope you are doing well

You don't have to worry if your allotment letter is missing, since you got the allotment letter it means you are already eligible to get admission in that respective college.

If you got college through online counseling and also downloaded the letter from their website you can contact the authorities through the helpline number mentioned on their website.

If you don't get any solution by contacting the counseling authorities through helpline number, you should go to the college immediately and tell them the whole situation you will be given another letter or maybe the college will provide some other solution.

Hope this helps