Why am I unable to create an invoice with a combination of taxable and non-taxable items?

As per the rule 46A - Invoice-cum-bill of supply, you will not be create an invoice with taxable and non-taxable line items for registered tax payers.

However, you can create an invoice with just the taxable line items and create another invoice with the non-taxable line items in Zoho Books as a workaround.