Mobile Enrollment policy should be assigned to only users.
On the Getting Started with Mobile Management page, navigate to the Enrollment Policy section, click Assign Policy to display the Assign Policy wizard, then click Add. Alternatively, from the left hand side pane in ZCC, navigate to Policies. Select a policy and click Action > Assign to User.
In the Select Object dialog box, browse and select the users to whom you want to assign the policy, click OK to add them to the Users to be Assigned list, then click Next.
On the Select Object dialog box, browse for and select the policy to be assigned to a user, click OK to add them to the Policies to be Assigned list, then click Next.
Review the summary page and click Finish to complete the assignment.