how can i use a variable inside a fetch from coinplayer api

I have a variable userselectedcrypto[message.author.id]; (discord.js) and I want to use that when getting info out of the fetch from coinplayer api let userdata = data.rates.userselectedcrypto[message.author.id]; <== like this, this however doesnt work, because it looks for something named that in the data variable that i get from my fetch... How can I fix this?