Standard deviation - New average

Hello Guys

I have one problem is about calculating average according standard devitation. I have several results, where I need to cut out the results outside the standard devitation "line". I'm trying to use average and countif but it doesnt work for me
The basic calculation principle is to find if the value is in SD range if not use False / 0 and with averageif ignore "zero" results. I've also tried a countif but it's still not working for me.

Can you help me please why this average did not work in formula?

I would like to use only one formula without additional cells.

See more in worksheet.

Thank you for any help

Kind regards
Andrej