If we go to:
/jenkins/blue/organizations/jenkins/standalone-project/activity
There are two buttons:
[ Run ] [ Disable ]
above the Status/Run/Commit/Message/Duration/Completed/(replay) table

If we go to
/jenkins/blue/organizations/jenkins/GitHub%2Frepo/activity

The Run & Disable buttons are missing. Disable being missing is understandable as the repository was discovered by project discovery. Run being missing is unfortunate.

/jenkins/job/GitHub/job/repo/job/master/ has a [ Build ] or [ Build with parameters ] button (depending on whether or not the job supports parameters) – And for the standalone-project the Run button works as Build or Build with parameters as applicable.