TL;DR: it states on mlagents documentary that the minimum version of unity editor supported is 2021.x. This conflicts with the current version of unity editor (2020.x). Therefore, to get the best support and experience in using mlagents, both mlagents and unity editor version are upgraded to at least the minimum unity editor version and mlagents package versions.
For the best compatibility between unity-side of things and mlagents, both platforms needs to be aligned in terms of their current changes as per mlagents guidlines: https://github.com/Unity-Technologies/ml-agents/blob/main/docs/Installation.md