When a CLM user tries to link a CLM artifact to a Windchill artifact, if the desired link type is not available in CLM, this is the sign of a faulty project association.
This is certainly because the link type chosen to link the CLM artifact with the Windchill artifact is not available for the current project association.
When facing this problem, you've to ensure the CLM project area is associated to the Windchill product or part with the correct association type, this will render the correct link type availables.
When following steps in page Configuring CLM Projects, carefully choose the desired association type.
Now, redo the same linking operation between a CLM artifact and a Windchill artifact, and choose the desired the link type.