Quick Launch missing from SharePoint subsite after deleting links

Most likely, the Quick Launch disappeared because all navigation items were removed from that subsite. When the Quick Launch is completely empty (no links or labels), SharePoint Online stops rendering the left navigation and hides the edit options, even though Quick Launch is still enabled in Site Settings.

This only affects the specific subsite where the navigation was cleared, which is why other subsites are unaffected.

Try go to Site Contents → Site Pages on the affected subsite -> Edit any page -> Select Add to navigation -> Save. If did'nt help try Settings → Site information → Change the look → Navigation -> Turn Site navigation visibility off, save -> Turn it back on, save and than try again Site Contents → Site Pages on the affected subsite -> Edit any page -> Select Add to navigation -> Save.

Hope this helps :)