Hello viraj,

if you add customer on orders

1.add customer account (customer profile – accounting tap - account receivable)-

or check

As per the error message, it is raised when account_id is not set when creating the invoice line. So please make sure that the accounts are set for the product's/product categories.

You are getting this error because the account is empty in invoice line, the account is filled automatically by default from product if you set expense account in product and if it's empty then it check if the product category expense account and it's empty then it take the Vendor Bills journal default account. 

So you have to set the account from invoice line and if it's not visible for the user you can give the user "Show Accounting Features - Readonly" permission from user security.