I downloaded
https://download.brother.com/welcome...6677ug10en.pdf and on page 64 it says :If you press the stitch selection key or do the operation correctly while the error message is displayed the message disappears." So while you may not have an "OK" key, perhaps if you just select the stitch you want to use, it will clear the "E6" provided there really isn't a tangle or something else.
Janey - Neat people never make the exciting discoveries I do.