This error appears if when you have used the \maketitle command, but have not specified any \author. An example of this is shown below:
%In your preamble \title{On the General Molecular Theory of Heat} % In the body of your document \maketitle
main.tex, line 5
You can solve this error by including an author:
%In your preamble \title{On the General Molecular Theory of Heat} \author{A. Einstein} % In the body of your document \maketitle
If you would not like to have an author specified on the title page, you can define \author{...} to be blank as shown below
%In your preamble \title{On the General Molecular Theory of Heat} \author{} % In the body of your document \maketitle
This will still leave some whitespace on the title page where the author would usually be written. If you would like to remove this whitespace, check out the titling package.