Thank You for posting in Q & A.
Your question is already answered in Q & A forum
refer - https://learn.microsoft.com/en-us/answers/questions/392905/not-able-to-choose-stotage-account-when-creating-f.html
For more details on function app with storage account.
refer - https://learn.microsoft.com/en-us/azure/azure-functions/functions-create-function-app-portal
https://learn.microsoft.com/en-us/azure/azure-functions/storage-considerations
If the Answer is helpful, please click Accept Answer
and up-vote, this can be beneficial to other community members.