Unable to login to SharePoint designer app

Issue:

When trying to login to the SharePoint Designer App, getting the below error:

Resolution: 

• Please check from the SPO admin center if the custom script is disabled. If it is disabled, enable it. Below are the steps:

1. From the SPO admin center, click on settings then click on Classic Settings Page as shown in the figure below:

2. Scroll down on Classic Settings Page and check the custom script settings as shown in the figure below:

If the above is already enabled, then please create a registry with the value defined below:

• HKCU\SOFTWARE\Microsoft\Office\15.0\Common\Identity\EnableADAL - REG_DWORD - 1

For more details, refer to the below Microsoft article:

• Enable Modern Authentication for Office 2013 on Windows devices - Microsoft 365 admin | Microsoft Docs