Weekly time periods can be created in the Classic UI but not able to be viewed in the Modern UX.
Monthly time periods can be created in the Classic UI, but not able to be viewed and modified in the Classic UI.
Release : 15.9.2
Component : CA PPM SAAS FINANCIAL MANAGEMENT
This is working as designed.
When trying to view any time period other than the Monthly, Quarterly, or Annual , the following message will appear:
13 Periods, Weekly and Semi Monthly cost plans are not supported. Please go to Classic PPM to view them.
This is described in the documentation Edit Cost Plan Settings