I don't agree with you completely 和 I completely don't agree with you 的意思不一样。

I don't agree with you completely. = 我并不完全同意你。（completely 在否定范围之内）

= I don't completely agree with you. = 我并不完全同意你。（completely 在否定范围之内）

英语理论上可以说：

I completely don't agree with you. = 我完全不同意你。（completely 在否定范围之外）

但实际上好像 completely 很少位于否定范围之外，要表示“完全不同意”，可考虑改用其他说法：

I completely disagree with you. = 我完全不同意你。

I don't agree with you at all. = 我完全不同意你。