Hello and welcome to the forum 
Which version of krita are you using and which operating system?
Please see this section of the manual:
Onion Skin Docker — Krita Manual 5.0.0 documentation
and this reply to another topic along with its associated screenshot:
Onion Skin isn't showing - #8 by AhabGreybeard
If you still have problems, please upload a full screen screenshot with the Layers docker, Animation Timeline and Onion Skins docker showing.