If you have enabled custom script at tenant level, just run the commands provided in the other Answer (Set-SPOsite <SiteURL> -DenyAddAndCustomizePages 0) to enable it at site level.
A quick method to check if the custom script on site level is enable: Navigate to one of your lists and then switch it to classic experience, see if you can see some of the ...
The global setting which you have applied takes up to 24 hours to activate so, just wait for that.
But if there is an urgent need, then you can execute the below PowerShell scripts.
Connect-SPOService -url https://<tenantname>-admin.sharepoint.com
Set-SPOsite https://<siteurl> -DenyAddAndCustomizePages 0
Immediately, the effect will be reflected,...