The F3 button for locking the sketch plane is the same preconfigured to increase the screen brightness on my laptop. Therefore, even within solid edge, trying to lock the sketch plane by pressing F3 ends up increasing screen brightness.
You could use the on-screen method......select the action you want to start with, then mouse over the plane or planar face, finally, mouse over and left click the padlock. Should be good to go.
Are your function keys alternative [ALT] selections [or FN key as@Tusharsays] rather than primary?....hence you screen brightness is first option on that key. If this is the case, you'd need to press Alt+F3 to execute this.
Sean Cresswell Design Manager Streetscape Ltd Solid Edge ST10 [MP8] Classic [x2 seats] Windows 10 - Quadro P2000