The 'Type' may be inactive in code tables, to activate them:
Each of the membership types need to be active for them to be available for transactions. All or one of the types may be inactive.
  1. Go to Administration > Code Tables
  2. In the Category drop-down, select Membership and click Apply
  3. Click Membership types
  4. Click the drop-down next to the type and click Edit
  5. Unmark the box "Inactive" and click OK

Your membership level may not have types assigned to it:
Membership types need to be connected to each of the membership levels. A particular membership level may not have membership types associated with it.
  1. Select the Memberships tab
  2. Click your the name of your membership program
  3. Select the Terms and levels tab.
  4. Highlight the level in question and click Edit Types
  5. Add the membership types that you need
  6. Click Save