GetMuxedStreams() does not contain definition for GetWithHighestVideoQuality()?

[ThomasBeHappy]

it's telling me
'IEnumerable' does not contain a definition for 'GetWithHighestVideoQuality' and no accessible extension method 'GetWithHighestVideoQuality' accepting a first argument of type 'IEnumerable' could be found (are you missing a using directive or an assembly reference?)

What am I doing wrong here?

[Tyrrrz]

Highlight the red underlined code, press Ctrl+., press "Add using statements" or whatever