Please note that this article refers to the old Salesforce integration for GetAccept. The new one is available here: https://appexchange.salesforce.com/appxListingDetail?listingId=a0N3A00000E1Z2sUAF
Turning On Orders
In longer established organizations (that existed pre-spring 2014 and using Salesforce) orders are not automatically enabled.
If that is the case follow this:
1) Click Setup in the top right corner (Salesforce classic)
2) Use the Search box and search "Order Settings"
3) Make sure "Enable Orders" is checked
Enable this, save it, then try to install again.