Did you read the error message? It tells you what the problem is.
Did you read the STACK install instructions? https://stack2.maths.ed.ac.uk/demo2018/question/type/stack/doc/doc.php/Installation/
Did you read the STACK install instructions? https://stack2.maths.ed.ac.uk/demo2018/question/type/stack/doc/doc.php/Installation/