unity降低版本从2019.3.0a7 to 2018.1.8f1后，报错com.unity.2d.sprite: Package [com.unity.2d.sprite@1.0.0] canno

we had a similar issue when downgrading a project from 2019.3.0a7 to 2018.1.8f1. This was for a 3D project, not a 2D project. Close Unity and open the location of your project and go into the Packages folder and rename the manifest.json file to manifest.old. Create a new project (we used the new “3D with extras” template from Unity Hub. Close Unity and copy the manifest.json file from the new project and paste it into the Packages folder of the project with the error. Relaunch the Project with the error and it should open without error. If there are errors, then you can delete the new json file and rename the original.

The alternative is to make a backup of the json file and then delete the lines in the manifest.json file with the packages that can’t be found, ie:

com.unity.2d.sprite
com.unity.2d.tilemap
com.unity.modules.androidjni
com.unity.package-manager-ui

原文出处：https://forum.unity.com/threads/package-manager-strange-error.659809/