Shape builder tool + Alt key not give the eraser mode

[AshanT]

when I choose the shape builder tool to subtract a shape from a collective shape, I usually press Alt and do that. But now it works as a duplicate function, it shows two arrowheads one is black and the other is white. I am ffrustrated to find a solution but. there is no issue like that I can find on the internet. please, someone, help me

[Monika Gause]

And the shapebuilder tool is selected?

You don't press any additional keys other than ALT?

[AshanT]

no, not purposely. 

[John Mensinger]

Have you tried closing and relaunching Illustrator? Resetting preferences? Rebooting the computer?

[AshanT]

yes done all those things. 

[Monika Gause]

Are other applications running? Google Chrome perhaps?

Other apps might steal modifier keys. Reboot the computer and launch only Illustrator - does that work?