error CS0234: The type or namespace name ‘UI‘ does not exist in the namespace ‘UnityEngine‘

1. After opening the project, an error is reported: error cs0234: the type or namespace name ‘UI’ doors not exist in the namespace ‘unityengine’

2. Solution: close untiy, delete the Library folder, and then reopen the project

[1] Close the project and delete the folder library. The location is as follows:

[2] . reopen the project and unity will automatically rebuild the relevant DLL

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