It would depend on the mission as they all vary. Simple missions using default aircraft & scenery should be no problem. More complex missions require 3rd party aircraft & scenery. These might be included but often need downloading separately, sometimes from various sites.
Missions will work without extra scenery installed. The most common cause of error messages with missions is missing or incorrectly installed 3rd party aircraft. These must be installed with their original folder names. Make sure you read the install instructions carefully. These are not always easy to follow.

I'm talking about missions posted on this site. There is no standard example of a mission. Some are very simple & use default aircraft & scenery. Others require extra 3rd party aircraft & sometimes scenery. These files may or may not be included in the download. Please read the author's install instructions carefully. Most downloads will contain instructions in a file named Readme.txt or similar. These can be opened in Notepad or Wordpad.

To install a basic mission, unzip the downloaded file & paste the MIS & possibly DYN files into the CFS2\MISSIONS folder.