How to Fix Excel Keep on asking for a key

I have downloaded microsoft office. I am trying to use excel and every time i use it keeps prompting me to sign in or use a product key. I have done multiple times and am yet still unable to use excel. I tried to download office on a different device but i remain unsuccessful as even when i sign in and use a product key it still shows "Excel (Unlicensed Product)", and after a while a pop u coms saying that there is a problem in excel. When i close this prompt or even when i try to fix the situation, Excel just closes.And the same thing happens all over again. This is getting really frustrating and annoying.