what is the significance in the last sentance "This being you mudt create"?

The creature says,

"You must create a female for me, with whom I can live in the interchange of those sympathies necessary for my being. This you alone can do; and I demand it."

The creature demands that Victor ease his suffering.

The monster wants a mate and he sees that mate as his only possible solution to the loneliness and bitterness that he feels. He wants Victor to somehow provide for him that which Victor seems to have.