The catalog task does not get created from the workflow activity if the catalog item consists of a Multi-Row Variable Set.
The activity gives a null pointer exception and the flow gets ended without the catalog task getting created in the RITM.
If variables Global field is set to "True" inside MRVS, Then this issue occurs.
Making the Global field to 'False' for all the variables that are inside MRVS should fix the issue.
As per this KB, You cannot set Global as True for any variable that belongs to a multi-row variable set.