Before we start to talk about the error, let me introduce a Script component editor property SynchronousInputID first. In Microsoft document, described SynchronousInputID as below:
The SynchronousInputID property has a non-zero value only in transformations with synchronous outputs. If the value of this property is zero, it means that the output is asynchronous. For a synchronous output, where rows are passed through to the selected output or outputs without adding any new rows, this property should contain the ID of the component’s input.
Which means if output data only one line, then the output columns are just part of the Input Columns, and there is no need for Output0Buffer. If your output data has many rows, then you need to create new Output0Buffer.
In SSIS, SynchronousInputID default value is zero. If you created a new Script component and have change this default value, but in C# script you are try to use Output0Buffer.AddRow(); to create multiple rows output, then you will get error message:
Output0Buffer does not exist
If you got this error, just follow below steps to change the SynchronousInputID value to non.
clicking on the Script Transformation Editor -> clicking the catagory Inputs and Outputs -> clicking on Output 0 -> changing SynchronousInputId to None in the right hand panel