Private methods are not visible in Test Classes. Hence use @TestVisible annotation so that the method is accessible for running tests only. Use as shown below.

    @TestVisible private static void processActionPlan(Workbook workbook) {
    
    your code 
    
    }

Refer this link [TestVisible Annotation](https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_classes_annotation_testvisible.htm) for further details.