Hi qixiong Li,
Please first check you edition of the SQL Server, SQL Server Express Edition is a free version that does not include the SQL Agent.
SQL Server Agent has to log on as Local System Account. Open up services console (start > run > type services.msc) and locate "SQL Server Agent" service and double click on it. Go to "Log On" tab and ensure "Local System Account" is selected. After confirming, you should be able to start this service.
Regards,
Zoe Hui
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