You've not told us which device (PC? Mac? iPhone? Android device?) you're accessing Facebook on. You've also not mentioned the web browser which you're using. (Internet Explorer? Edge? Firefox? Chrome? Safari? Opera?).
However the problem normally shows up when the website/service that you're trying to access doesn't trust the security settings of your browser. (It seems to be particularly common when using Chrome). So you need to ensure that you've updated your browser to the latest version or, possibly, switch to another browser.