First, on the Manager Screen - Manager Options - Team Control Settings, make sure that Scouting is set to your control, not your assistant.
Scouting in the Manual:
Go to Team + League Controls - Team - Scouting and make sure the Assignments option is selected in the dropdown on the right.
Look for coaches who have nothing listed under Current Job.
Right click the coach you wish to assign, then use the menu that comes up to assign them to a league or region, what they should look for, and how long they should look. Again, the
manual will be a good resource.
For assistant coaches assignment:
Go to Team + League Controls - Team - Personnel
You shouls see the list of people who work for you, with the coach assignments on the bottom.
If you right click on an assistant coach's name, you should get options for release assistant, or assign to one of 4 roles.