Hi @Camha Nguyen,
Thank you for posting in this community.
In this article: Unregister a site as a hub site,
This means that the old navigation bar in your site should be deleted as normal. It is very likely that your problem happened by chance. Therefore, I suggest you try the following:
1.Are other users facing the same problem? View through another computer or account.
2.You could try to clear browser cache then restart it.
3.Try to use a different browser or your browser's InPrivate mode to see if the same error occurs.
4.Contact your SharePoint administrator to download SharePoint Online Management PowerShell, and then use the PowerShell command to unregister the site as a hub site. Please replace your site name and tenant name.
Connect-SPOService -Url https://contoso-admin.sharepoint.com
Unregister-SPOHubSite -Identity https://contoso.sharepoint.com/sites/Marketing
If it is successful, it will be like this:
If it is wrong, it will report an error, so that we can also check whether the original site has been successfully unregistered as the hub site.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.