What add did you get? Can you provide a screenshot of it?
I rather not to use this ANSWER unless someone ensures me why this abuse of Apple Community and how get rid of the this annoying ADD!
Are you seeing the add on other sites? If so download and run the free version of Malwarebytes. It was developed by a long time contributor to these forums and a highly respected member of the computer security community.
Also download and run Etrecheck. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.
Copy the report

and use the Add Text button to include the report in your reply. How to use the Add Text Feature When Posting Large Amounts of Text, i.e. an Etrecheck Report
IMPORTANT:
Before running Etrecheck assign Full Disk Access to Etrecheck in the app's Privacy preference pane so that it can get additional information from the Console and log files for the report:

ALSO IMPORTANT:

Then we can examine the report and see if we can determine the root of the problem.
