In order for a package or refund credit to be able to be used as payment for a session the following must hold. Work through the following to resolve:
The session must have the relevant package type (bundles, passes and/or memberships) selected as an accepted Payment method. Edit the session to check and update as needed.
The session must have the same tag as the relevant package. You can check the set tag for the bundle/pass/membership in Packages>click on package name from appropriate filter>review table in Details page. Edit the session on the Schedule page to check and update the tag as needed.
The package must have Sessions set as a purchasable object. Edit the bundle/pass/membership from Packages>click on package name from appropriate filter>Edit this package to update as needed.