If the data set is not considered a “good data set” then the file will not be able to evaluated for Reliability. A good data set if defined by variability.
Students: If there are not enough students this could trigger an error message; “Assessment could not be evaluated.”
Scores: If an assessment scores have too many students getting the same question (item) correct OR every student gets a specific item correct this could trigger an error message; “Assessment could not be evaluated.”
Remedy this by removing a failing question (the question that is not yielding a diversity of answers) if that is the source of the problem. If the issue is that you don’t have enough questions or enough students, try combining assessments together to yield a more hearty data set.