The CIS feature needs to be toggled on to allow you to add subcontractors or edit existing suppliers to become a subcontractor.
You need to set the supplier as a subcontractor before you can create bills with CIS deductions.
Navigate to the Contacts > Suppliers menu and click on the Add supplier button.
Fill in the required details of the subcontractor on the Basic information tab.
Click on the CIS Subcontractor Details link in the box to the left.
Update the CIS subcontractor drop-down box to Yes, make sure to update the Type drop-down list appropriately and fill in the other details as appropriate.
Once complete click on the Save button.
Please note: You can edit an existing supplier and update the CIS subcontractor details as shown in step 4.