This is most likely due to the fact that the transaction application (donation campaign, teamraiser event etc.) has not been associated with a receipt or the receipt is in unpublished status. First, verify which campaign and in which application the transaction occurred, then use the following steps to generate the receipt:

If the object is not associated with a receipt:
  1. Go to Setup > Receipt Manager.
  2. Select edit next to the receipt.
  3. Go to step 3, Select Applications.
  4. Select Configure next to the transaction application type.
  5. Associate the object with the receipt by adding it from the left hand column to the right hand column. Save your changes.
  6. Navigate to the constituent record and the Generate Receipt link will appear.
  7. Select the link and mark the radio button for the appropriate notification action to the constituent.
If the object is associated with an unpublished receipt:
  1. Go to Setup > Receipt Manager.
  2. Check the receipt Status column for any receipts that may be in "Unpublished" status. Select Edit next to any "Unpublished" receipt.
  3. Go to step 3, Select Applications.
  4. Select Configure next to the transaction application type.
  5. Verify that the transaction application type and the campaign is associated with the Unpublished receipt.
  6. Select the blue Receipts tab and return to the receipt list.
  7. Select the Publish link next to the receipt you just verified.
  8. Navigate to the constituent record and the Generate Receipt link will appear.
  9. Select the link and mark the radio button for the appropriate notification action to the constituent.