I don't get it a new mattress

A mother has just bought a new mattress for her son

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Mom: Why is he making such a fuss? Coming.

Uh, that's not how you make a bed.

Boy: It isn't? Didn't I make it properly? Uh, Well, anyway,

I can't sleep in this bed. It's too lumpy.

Mom: Too lumpy?

Boy: Yes, see that big lump.

Mom: A lump! I don't get it a new mattress.

Get off the bed for a second. Why, it's a piggy bank.

What's this doing under the mattress?

Boy: Uh, I forgot. I hid it there.

Mom: You hid it there?

Boy: Hum.

Mom: Why did you hide it there of all places?

Boy: To keep my money safe.

Mom: In a piggy bank? But this is just a toy.

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What does "I don't get it a new mattress" mean?

Maybe it needs a comma after “I don't get it".

>>> I don't get it, a new mattress.

Thank you