This could be due to PDF receipts not being enabled, or the transaction application (donation campaign, teamraiser event, etc.) has not been associated with a receipt, the receipt is in unpublished status, or the receipt was associated with a different transaction type than the transaction itself. 

Verify which campaign and in which application the transaction occurred, then use the following steps to generate the receipt:
  1. Navigate to Setup > Receipt Manager
  2. Under the Actions column, next to the receipt name, click Edit
  3. On the left hand navigation pane, select Select Applications
  4. Click Configure next to the appropriate application type
  5. Associate the object with the receipt by adding it from the left hand column 'All Selections' to the right hand column 'Your Selections'
  6. Click Save
  7. Then follow the steps below to generate the receipt
If the object is associated with an unpublished receipt, publish the receipt:
  1. Navigate to Setup > Receipt Manager
  2. Check the receipt Status column for any receipts that may be in Unpublished status
  3. Under the Actions column, click Edit
  4. On the left hand navigation pane, select Select Applications
  5. Click Configure next to the appropriate application type
  6. Confirm the campaign, store, or ticket displays under the 'Your Selections' column
  7. Click the Receipts tab to go back to the list of receipts
  8. Under the Actions column, click Publish
  9. Follow the steps below to generate the receipt
If the object type does not match the receipt type (online or offline) at the time the transaction was entered:
  1. Navigate to Setup > Receipt Manager
  2. Under the Actions column, next to the receipt name, click Edit
  3. On the left hand navigation pane, select Identify Template
  4. Scroll down to the Types of transactions configuration
  5. Choose the type of receipt that matches the transaction type (Online and offline transactions, Online transactions only, or Offline transactions only)
  6. Click Save
  7. Follow the steps below to generate the receipt
How to generate the receipt:
  1. Navigate to Constituent360 > Constituents
  2. Enter the name of the constituent
  3. Click Find
  4. Select the Transactions tab
  5. The Generate Receipt link will appear under the Actions column
  6. Click the link and mark the radio button for the appropriate notification action to the constituent