When the coupon link in your campaign email goes to an Error Page, there are three most possible reasons for that
1. The Link Protocol isn't set to <Other>
The link protocol is set when hyperlinking the Coupon Code to a text in the email. Make sure that the protocol is set to <Other>
If other protocols are selected while using a Coupon Code, the link will not work properly and could cause the link to go to an error page.
2. You entered or pasted the View Page URL of the Coupon instead of its Coupon Code (Page Shortcode)
The View Page URL only works for Previews. You can't even redeem the button when previewing the coupon. Make sure to link or insert the Coupon Page Shortcode to the email.