After entering runway for takeoff , "Takeoff Power Set" (aka. Takeoff) is not detected.

In some airports runways are not detected as runways in MSFS 2020. Additionally, runways that look like the image below is sometimes not detected as a runway. If your runway looks like the image below, keep taxiing till the actual runway start, which is usually indicated by the runway number printed on the runway.

(KLAS - R26R) NOT considered a runway (so taxi more)

(KLAS - R26R) NOW considered the runway

This behavior is not consistent throughout MSFS 2020. Sometimes the same runway which is not recognized as a runaway in one instance will be recognized in another instance.