what does SI not working mean when I try to clear open apps

[AC Question]

S4 galaxy phone. .WHENEVER (last three days) the center button at bottom of phone is depressed, the following msg appears.."Unfortunately, SI is not working" & I have to choose. .OK.. trying to close open apps..after about 30 seconds I can continue to use phone. ..

 

[Javier P]

Welcome to the forums. Do you mean System UI? See here.

https://forums.androidcentral.com/showthread.php?t=763338
How can I fix the "com.android.systemui has stopped" error?