You see the message 'Matching user not found' when the username does not match between the Okta app assigment and the Salesforce user. Verify that your provisioning credentials for Salesforce are valid by Testing the API Credentials. This provisoining message can be resolved by the steps below:
Edit the task on the dashboard. Make sure the Username and attributes match the user on the Salesforce side.
If this is a new Salesforce user, enable the Create Users settings under Provisining for the app within Okta.
If this is an existing user, enable Update User Attributes under Provisining for the app within Okta. (Warning: This option will make Okta the profile master and update all information in Salesforce for that user.)