I am unable to add 'pay now' to invoices. There is no payment tab either in my account GUI. What should I do?

Hi there, @godsbrainy-gmail.

Are you trying to accept electronic customer payments in QuickBooks Online? If so, this feature depends on your QuickBooks region/country. 

If your customers are unable to see the Pay now option, that means the online payments for the invoices isn't available for your country. 

I'd suggest following the recommended steps given by Fiat Lux - Asia. Just go open your QuickBooks Online account and go to the Apps menu. From there, search for the app.

I'm also adding this resource to help you manage invoice fees in QuickBooks: Record invoice payments in QuickBooks Online.

Keep in touch if you have other questions about handling invoices in QuickBooks. I'll be happy to lend a helping hand. Stay safe always.