There is not any immediate procedure option for calculating the KR21, which happens to be determined by the (usually unrealistic) assumption that all things in the scale have equal issues stages (i.e. equal proportions of 'successes' to the things). The KR21 can be calculated in SPSS by using the AGGREGATE https://kr2005938.full-design.com/kuder-richardson-reliability-an-overview-69479162