Reality "Engine Content" moved into "Plugin Content" since Reality 2.10 is Plugin. When you convert your projects which has older version than Reality 2.10 you should update your assets. Convert your old project with Reality 4.26 Editor and once it is complete you will see a message log like this, press "x" to close.
You will recognize that there is no visible material on your object
Open your material. You will see some errors due to Reality Engine content direction has been changed.
Let's re-select the same content but this time from the Plugin Content.
You should enable "Show Plugin Content" to see Reality contents.
Select "ZDNoVideo". (as an example for this project)
Click on "Save".
All completed and ready to use.