When you call GetRoleByName, it is returning a RoleInfo object.  If that role doesn't exist, the RoleInfo object will be Nothing.  So, before you ask for its RoleID, make sure it isn't Nothing.

Dim role as RoleInfo = objRoleController.GetRoleByName(RoleName)
if role IsNot Nothing then
...
else
'create role
end if

DotNetNuke.Security.Roles.RoleController().GetUsersByRoleName() seems to be your best bet for getting the users of RoleB assigned to RoleA.  You'll get an ArrayList of UserInfo objects.  You can access each item in the ArrayList with

For Each user as UserInfo in usersList
'call AddUserRole() for the current user object

..
Next

Hope that helps,

It's happening on the line:

Dim roleId As String = objRoles.GetRoleByName(PortalSettings.PortalId, CoachRoleName).RoleID

You can't call .RoleID is the role is Nothing.  It's here that you need to, first, get a reference to the RoleInfo object, second, test if the role IsNot Nothing, and, only then, ask for its RoleID.  Does that make sense?

You need to assign the RoleInfo object returned from GetRoleByName into its own variable, and test if it is null before you call .RoleId.  You do not have to assign a RoleId to the new role.  After your save the new role object, it should be assigned a RoleId.  I've reworked your code a bit for you.  I added checks for null on both the user and role objects, consolidated some duplicate objects that weren't needed, and added the use of Me.PortalId instead of 0 (you never know when your code will get used in a way differently that you originally intended, so it's best to be safe, especially when PortalId is so easy to get).

Protected Sub CreateButton_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles CreateButton.Click
  Try
    Dim objRoles As New RoleController()
    Dim UserName1 As String = Coach.Text
    Dim CoachRoleName As String = (SchoolNS.Text & "-Coach")
   
    Dim user as UserInfo = UserController.GetUserByName(Me.PortalId, UserName1)
    if user Is Nothing Then
      Return
    End If
   
    Dim userId As Integer = user.UserID
    Dim objRoleInfo as RoleInfo = objRoles.GetRoleByName(PortalSettings.PortalId, CoachRoleName)
    if objRoleInfo Is Nothing Then
      objRoleInfo = New RoleInfo()
      objRoleInfo.AutoAssignment = False
      objRoleInfo.BillingFrequency = "Month"
      objRoleInfo.BillingPeriod = 6
      objRoleInfo.Description = SchoolNS.Text & "Coach Role"
      objRoleInfo.IsPublic = False
      objRoleInfo.PortalID = Me.PortalId
      objRoleInfo.RoleName = CoachRoleName
      objRoleInfo.ServiceFee = "0.00"
      objRoles.AddRole(objRoleInfo)
    End If
    objRoleController.AddUserRole(Me.PortalId, userId, objRoleInfo.RoleId, Null.NullDate, Null.NullDate)
    errorLabel.Text = "Completed"
  Catch ex As Exception
    errorLabel.Text = "ERROR: " & ex.Message.ToString()
  End Try
End Sub

Hope it helps,