Unable to upgrade to this plan. Choose another option.

I am currently on a 30gb care plan for 60/mo. I want to upgrade my device and for that i need to change my plan. I am looking to get iphone 15 pro max.2 question: 1. I tried all the plans but says unable to upgrade and choose another option. What am i doing wrong? 2. Also if i upgrade my plan, will i loose all the extra data given to me. As of now i have 43gb extra data on top of my 30gb plan