Workflow updating a multiple value lookup column

I have a couple of task lists for a project. One list is the list of main projects, the second is a list of tasks under each main project.

The main projects has a lookup column that allows multiple values showing all tasks assigned to it.

The task list has a lookup column that says what main project it is attached to.

Currently both of these fields have to be input manually.

I’m trying to write a workflow to check if a task is assigned to a project, and add it to the lookup column of the main project.

I’ve tried associating the workflow with the task list so that every time a new task is created it will add it to the project, but I am having problems actually getting the workflow to run successfully.

The workflow is currently only one step, update list item.

This is linked to the parent task list, and the field it is updating is the lookup column showing all child tasks. It is attempting to update this field with the title of the child task.
The find list item is looking for the title matching the lookup value of the parent task for the child task. This is because people creating child tasks should be filling in the parent task on creation.

The workflow runs and give back an error that, “The workflow could not update the item, possibly because one or more columns for the item require a different type of information.”

How could I change the workflow in a way that lets me keep the lookup column in the parent task list and allows the workflow to run.