Apparently the screenshot upload was successful...hopefully it will aid in resolving the issue. If it helps, the customer data was imported from an external .xls file
Sounds like a frequent user mistake and one that I have suggested to FileMaker that they deal with by adding some more documentation with their starter solutions. My guess is that you did not enable auto-enter options (controlled by a check box in a small funky dialog that pops after you click "import") and thus your customer records do not have the needed ID.
See this thread for more, including a way to fix this without having to re-import this data: Help importing Excel data into a Filemaker Starter solution
This is for a different file than yours, but the issue and the possible solutions remain the same.
Thank you PhilModJunk; problem resolved. It's rare that I miss checking this box when importing but may've overlooked this time.