ADB Backup does not work for Skype App
I Think you need to connect  with pc and need to take backup as mentioned in the skype doc 

this is not poosbile with android 

https://support.skype.com/en/faq/FA413/how-do-i-back-up-my-configuration-and-instant-message-history?platform=android

this is for windows 
https://support.skype.com/en/faq/FA413/how-do-i-back-up-my-configuration-and-instant-message-history