The service(s) may not be associated with the type of case you are trying to add it to
Click on Settings > Case Type then click on the type of case you are dealing with
Ensure that the Case Service is associated with that case type.
Here is a link that offers additional assistance if necessary: https://support.crosstrax.co/a/solutions/articles/22000241135-what-is-the-difference-between-a-case-type-and-a-case-service-/en