Our Power users are used to Sharepoint Designer and do some heavier customization, They have developed this skill while working with on-premise version of SharePoint.
Now as organization is migrating to Share point online so expectation is the same. We recently faced issue where users were using designer and were not able to customize the pages of office 365 Communication site. (Access denied error was received)
On bit of research it was found :
For Office 365 communication sites, add and customize permissions by default are denied but for classic team sites, it is allowed.
To resolve this launch share point online shell and run below command
Set-SPOSite “SharePoint site URL here" -DenyAddAndCustomizePages $false
You can check the result as well by running get command.
This will allow Communication site and pages to be edited in SharePoint Designer and error will disappear.
Thanks for reading