In the 'Agreements' tab, add 'Data Mapping Error' column to the view and check the error message. The creator of the Data Mapping Template would have also received an email with the error if the notification setting is turned on under 'Setup' > 'Develop' > 'Custom Settings' > 'Adobe Sign Settings' > 'Manage' > 'Enable Mapping Error Notification'.
Alternatively, you may even add the field 'Data Mapping Error' to the 'Agreement' object under 'Setup' > 'Create' > 'Objects' > 'Agreement' > 'Field Sets' > Click 'Edit' next to 'Additional Fields' > Drag and drop 'Data Mapping Error', 'Data Mapping Result' and 'Data Mapping' to the 'In the Field Set' box > 'Save'. You can then change the agreement object's 'View' page to 'Echosign Agreement Advanced' VF page under 'Buttons, Links, and Actions'. Once you change the VF page for 'View' to 'Echosign Agreement Advanced' VF page, open an agreement from the 'Agreements' tab, you can now see the data mapping fields (and the error) that you added on the agreement.