It seems as though you have your sidebar option set to
start. Open the Configuration Manager and see what the sidebar option is set to. It should be set to sidebar; if it is, create a page called sidebar and make some entries and close it. If you've already done that and it isn't working, then delete the sidebar option and leave it empty, and click save --this will disable the sidebar altogether and reset the cache. Empty the browser cache. Then restart the wiki; there should be no sidebar now. Go back to the Configuration Manager and and re-enter the name of a page for your sidebar option; it doesn't have to be sidebar. Then open your sidebar page and put in some entries. Then restart you wiki.
See
https://www.dokuwiki.org/config:sidebar.