Hi Yorgos,
Welcome to Sony Community!
We apologize for this inconvenience. The model VPCF23S1E is a non-US model thus we have limited information about this device. Though, you may follow the steps provided in this link to fix the issue on your keyboard, https://us.en.kb.sony.com/app/answers/detail/a_id/31749/p/48903,49902,57562,70867/c/65,66/kw/function%20key%20stopped%20working.
If the issue still persists, we highly recommend visiting our Sony Global Web site for information on contacting the Sony Support Center in your region at http://www.sony.net/SonyInfo/Support/ to assist you with this issue.
Regards,
Jen
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