I can’t update my phone with mobile data

What options do you get?

It would be better if you can please re-post with a screenshot, in order to understand the phenomenon better. Images/pictures speak louder than words. 

Please understand this is a public forum: While attaching a Screenshot, do not provide any personal credentials, such as IP address, Card detail, email id, apple id, IMEI number, Serial Number, Phone number, Order ID, if you are sharing a map ensure your location is not identifiable, etc.

With the limited information that you have provided, It is hard to even guess the problems faced by you. 

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Please re-post with detail so that someone else can help.

You need a 5G data connection to upgrade using cellular data. You can not upgrade using LTE.

Your original screen shot was 4G, not 5G. Try it again while connected to 5G.