This option can only be added to offline-only or online/offline donation forms. Here is how to verify this and also how to add checks as a payment type:
  1. Go to Fundraising > select Donation Management.
  2. Select the All Donation Forms tab.
  3. Click Edit next to the appropriate donation form.
  4. Scroll down to 6. Interaction Mode and make sure it is either offline-only or offline/online.
  5. Go to step 3. Design Donor Screens and click Edit next to Donation Form.
  6. In the box on the right-hand side, scroll down to the bottom of the list and select Payment Type. Now click Save Order and Edit Selected.
  7. Scroll down to option 7. Check Payment and enable "Yes, allow administrators to record donations received via check."