The error message "the data you are trying to access is unavailable" when running a mailing list report on a campaign using the NPSP Household Mailing List button likely indicates a permissions issue. Since the button works as expected for the system administrator, the issue is probably related to user permissions.
Apex Class Access (A): The most likely cause is that the user does not have access to the necessary Apex Class for Manage Households. Ensure that the user has the required permissions to access and execute this Apex Class.
Reference:
Salesforce Nonprofit Success Pack Documentation
Salesforce Help: Manage Apex Classes and Permissions

To efficiently enter and de-duplicate donor information received from a vendor's monthly spreadsheet, the NPSP Data Importer is the recommended tool. This tool helps in importing data and associating individuals with Opportunities and Campaigns while handling deduplication. Here's how to do it:
Prepare the Spreadsheet:
Ensure the spreadsheet contains columns for donor information, donation amounts, and solicitation campaign details.
Access NPSP Data Importer:
Go to the App Launcher.
Search for and open "NPSP Data Importer".
Configure the Data Importer:
Map the columns in your spreadsheet to the corresponding fields in Salesforce, such as Contact, Opportunity, and Campaign fields.
Ensure the NPSP Data Importer settings are configured to check for duplicates based on criteria like email or name.
Upload the Spreadsheet:
Upload the prepared spreadsheet to the NPSP Data Importer.
Review and confirm the data mapping.
Run the Import:
Run the import process.
The Data Importer will create new records and update existing ones, associating Opportunities with Contacts and linking them to the appropriate Campaigns.
Verify and Clean Up:
Use the NPSP Contact Merge tool to handle any potential duplicates that might have been missed.
Verify that all data has been imported correctly.
Reference:
"NPSP Data Importer" from Salesforce Help: NPSP Data Importer
"Managing Duplicates with NPSP" from Salesforce.org: Managing Duplicates