Okay ... some progress.
I removed the sl_id from the data array.
The table was updated, with new rows, but no data was inserted. I noted that the first field, sl_store, was blank, and all the others where I attempted to enter data contained NULL. I did enter 'USA' as the value for sl_country, and it was properly inserted. I used the single apostrophes around USA, and am wondering if it will be okay to put them around the php variables.
More to come.