A 266 Error Code indicates the output of the amplifier is not terminated in 50 Ohms. Information on other error codes can be found in the User Manual.   You can download manuals from Tek.com by going to the "Downloads" icon on the right side of the web page.  Select the download type as "manuals", enter in your instrument model number and then press "search".  At the search results use the filter on the right to filter for the manual type you are looking for.