Presentation error means the data outputted by the program is correct, but the seperators between each data tokens are mismatched with testcases. It is usually caused by extra white spaces or line breaks.
When a student makes a presentation error, the grade will be the product of the testcase's max grade and the ratio.
If you want to be strict, set it to 0% and a presentation error will worth zero.
If you don't mind such trival issues, set it to 100% and a presentation error will be equivalent to an accept.