Looks like the scripting capabilities is turned off in your site.
Whenever you create a new site, the custom scripting capabilities are turned off by default. You need to enable the custom scripting capabilities to have the ability to managed navigation.
Option 1: Go to SharePoint admin center > settings. In the settings page, in the Custom Script section, set the values to allow as in below image:
Now, you need to wait 24 hours for the scripting capabilities to be enabled. After that is turned on, you will be able to save the settings.
Option 2 : Using SharePoint Online PowerShell
After opening the SPO PowerShell, run the below commands:
Connect-SPOService -Url "https://tenantname-admin.sharepoint.com"
Set-SPOSite "https://tenantname.sharepoint.com/sites/test" -DenyAddAndCustomizePages 0
Once you have executed these commands, it will be reflected immediately and you don't need to wait for 24 hours.
Download link- SPO PowerShell:
PS - You require SharePoint admin credentials for the above mentioned steps. Site Collection admin wont work.