The error indicates that the Item in question hasn’t been assigned to the site via the “Cards > Inventory > Item Quantities” window.
To fix this go to “Cards > Inventory > Item Quantities” to open the “Item Quantities Maintenance” window as per below:
Once you are in this window do the following:
1) Select the Item
2) Click “Site ID”
3) Click the lookup glass (leave “Site Lookup” selected with “All”)
This will show the “Sites” window listing all the sites.
Finally select the site in question and click “Save” to assign the Item to the Site.
Please note you can also assign all items to a new (or existing) site via “Cards > Inventory > Site” and click “Assign”