Remember that you won't be able to cancel the contract once that you've
signed.
这个句子中的 once that 用错了吗?为什么呢?
你的句子中 once that 肯定错了呀,因为 once 在这里本身就是连词,引导时间状语从句,它后面不可以再添加连词 that 的:
误:Remember that you won't be able to cancel the contract once that you've signed.
正:Remember that you won't be able to cancel the contract once you've signed.
记住:你一旦在合同上签了字,那就不能取消了。