Enable SSO Login in APEX
Login to your Workspace
Click on Shared Components
On the Security section
Choose Authentication Schemes
Click on Create
Under Name section enter
Name: SSO
Scheme Type: SAML Sign-In
Under Settings section enter
Use SAML Attributes of: Instance
Convert Username To Upper Case: Yes
Click on Apply Changes
This SSO Authentication scheme which you created will enable the SSO login for your apps in the APEX workspace.