If the eReceipt button is missing, it indicates that either you are not editing a donation form part, the donation form part is using a Payment 2.0 part or eReceipt stack name and number have not been configured in the account. 

Confirm the part type is Donation Form
  1. Edit the part
  2. Click the Properties tab
  3. Confirm the Part Type is Donation Form
Confirm the payment option on the Donation Form
  1. Edit the part
  2. Scroll to Payment setup and confirm the payment method being used:
    • If Proceed directly to payment is selected, this indicates you are not using a payment part
    • If Use Payment part is selected:
      1. Note the exact name of the page in the dropdown
      2. Navigate to Site Explorer>Pages and templates
      3. Click the Search tab
      4. Search for the Payment Page from step a
      5. Click the Payment Page to select it
      6. In the lower pane, confirm the part name on the page. It will either say Payment or Payment 2.0. 
Confirm the reset eReceipt receipt name and stack numbers are configured. 
Note: if the eReceipt stack name and number is not configured, the eReceipt button will not be available. 

Additional Resources:
How to enable or disable eReceipts