as I hardly ever visit pinside, not due to lack of interest but due to lack of time, if you had contacted me directly thru the contact page of my web site (www.flippp.com), you would have received a quicker answer
the error codes are duly described in the manual at page 22, but not clearly, sorry for that:
[L] TEST + coil name + STUCK SWITCH
After firing a coil, one of its associated switches remains stuck permanently (for example a drop target that never stands back up).
-> Error only displayed if DIPSW+ 48 is enabled
so in your case, it's related to the coil driven by lamp output #17 = varitarget reset coil: one of the varitarget switches remains closed.
it is a common problem with varitargets in general, they don't always go back to resting position properly and/or the several contact blades are misadjusted or get dirty.