Add credit card declined instructions

doc/subscriptions/index.md

@@ -126,6 +126,19 @@ If you're purchasing a subscription for an existing **Core** self-managed
 instance, ensure you're purchasing enough seats to
 [cover your users](../user/admin_area/index.md#administering-users).
 
+### Credit card declined
+
+If your credit card is declined when purchasing a GitLab subscription, possible reasons include:
+
+- The credit card details provided are incorrect.
+- The credit card account has insufficient funds.
+- You are using a virtual credit card and it has insufficient funds, or has expired.
+- The transaction exceeds the credit limit.
+- The transaction exceeds the credit card's maximum transaction amount.
+
+Check with your financial institution to confirm if any of these reasons apply. If they don't
+apply, contact [GitLab Support](https://support.gitlab.com/hc/en-us/requests/new?ticket_form_id=360000071293).
+
 ## Manage your GitLab account
 
 With the [Customers Portal](https://customers.gitlab.com/) you can: