I'm experiences the same issue after importing unity 2d extras, my unity version is 2019.4.40f1 LTS.
After installing git and adding its path to the environment variables I'm try to import the package through package manager using link
(https://github.com/Unity-Technologies/2d-extras.git#1.5.0-preview) but its not working and my mainfest.json file under packages folder also missing
what should i do???