Hello,
First of all, be sure that the sidebar is enabled in Theme Settings -> Blog -> Blog layout.
https://prnt.sc/oCu9QUBXXhi_
Then you need to define some elements for the sidebar so navigate to Dashboard -> Appearance -> Widgets -> Main Widget Area.
https://prnt.sc/JzTladd5URF8
In the end, the sidebar will appear on the single blog post page.
https://gyazo.com/bfe3e98e95f998e63e099c79bbcb9aa8
If you have any other questions or concerns, please do not hesitate to reach out, I’d love to help!
Kind Regards