If you enable any only payment method like as Stripe, PayPal or Paytm, the payment options will show on the invoice preview page on the client portal. Online payment options are not visible in the admin portal.

You can also send the invoice link to the clients by email. In that case, the client can view the email without logging into the client portal. The online payment option is available on that invoice link as well.

To enable the payment link, without requiring the client login into the client portal, please go to Settings>Sales & Prospects > General > Enable the setting: Client can pay the invoice without login. Then modify the Send invoice email template and add the payment link {PUBLIC_PAY_INVOICE_URL} there.