Do you have a manual for your register? If so, take a look at pg. 143. This shows how to set the options for the % key. I suspect you have digit 6 (N6) set to 1 and that is causing the E15 when you try to use the % key. If you don't have a manual, post again with a description of how you want the % key to function and I'll give you specific instructions to set it up, if I can.
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