1. No credit card processors are set up since the webstore only accepts invoice payments.
2. The setting in Setup > Site Builder > Set Up Web Site > Shopping tab > Checkout Preferences section > Sales Order Type = Per Customer Basis.
The reason behind this error is because if the Sales Order Type = Per Customer Basis, the Credit Card Form & Invoice Form are now editable fields. This means that both invoice payments as well as credit card payments are now allowed in the webstore. But because there are no credit card processors set up in the account, this is what is causing the System Maintenance error-- the system trying to look for a valid credit card payment method.
In order to fix this, please do the following:
1. Navigate to Setup > Site Builder > Set Up Web Site.
2. Click on the Shopping tab.
3. In the Checkout Preferences section, set Sales Order Type = Standard Sales Order - Invoice form.
*Note: Alternatively, user can select any form that has been customized from the Standard Sales Order - Invoice form. It needs to be customized from this specific form, and not the other standard Sales Order forms.
4. Click Save.