This was the third attempt to send a sample return spacecraft to the Moon (after failures in June and July 1969). On this attempt, the spacecraft successfully reached Earth orbit, but failed to inject itself on a trans-lunar trajectory.
Later investigation indicated that the Blok D upper stage had failed to fire a second time for translunar injection due to a problem with a fuel injection valve that had become stuck during the first firing of the Blok D (for Earth orbital insertion). As a result, all the liquid oxygen in the Blok D had been depleted.
The Soviet press named the vehicle Kosmos 300 without alluding to its lunar goal. The payload decayed from orbit about four days after launch.
Siddiqi, Asif A. Beyond Earth: A Chronicle of Deep Space Exploration, 1958-2016. NASA History Program Office, 2018.