2 functions in xyplot

"Unexpected symbol" often means missing punctuation.

In this case, it looks like

panel=function(){panel.xyplot(Kalibrierung$Spannung,Kalibrierung
$Magnetfeld)panel.abline(reg=test)}

this argument is missing a semi-colon ";" before "panel.abline".

panel=function(){panel.xyplot(Kalibrierung$Spannung,Kalibrierung
$Magnetfeld); panel.abline(reg=test)}