What stage are you at? When you find her in the warehouse or when she's living in the safehouse?
If it's the first search for the warehouse, take a fishing rod a fish then give it to her.
If its the second then just go to new ark city and forget about the task and keep progressing and discover new places, going to the abandoned town and grinding by fighting the enemies by using the "seek" button can help you increase your status early game . after a while an event will start where the don captures you and ask about Cassie, that's when you'll be able to pick up the gain Cassie's trust task again....... If i remember correctly