Hi,
Found answer. Had the same issue when creating function app. After checking Dev tools I saw JS error:
"Cannot read property 'resource Id' of null at n_getAppInsightsResourceJson"
I checked out fields in the form and I left"storage account" and "application insight" fields to be created by default.
Storage account had name "(new)storageaccountxxx". To resolve issue I had to click on the link below field and manually create a storage account and application insight. After creating manually both of the fields, no error was found in the dev tools and function app was created successfully
Thanks