After importing an existing Android app project in Eclipse, I encountered the following error when trying to build:

■ Error Message

Cannot build project until build path errors are resolved

■ Solution

1. Open "Project Properties -> Java Build Path -> Order and Export tab"
2. Check the "Android x.x" checkbox and click "OK"
3. Build

[Reference]

ビルド・パスのエラーが解決されるまで、プロジェクトをビルドできません - のねの日記 - human brain programing

That’s all from the Gemba.