What is the cause of "Latex Error: The font size command /normalsize is not defined"

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I follow the instructions online to set up environment for LaTeX using Sublime text 3 and skim. However, I encountered a problem. When I try to compile the latex document shown in the picture, there are a lot of errors shown and the PDF generated is blank, i.e. without the "Introduction: ..." line.

Screenshot of ST3

And I downloaded all relevant program including latexmk.

System Check in ST3

Is there anything I can do to solve this problem?

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