[译] 你将会在一两天内收到她的信。
[误] You will receive her letter in one day or two.

[正] You will receive her letter in a day or two.
[正] You will receive her letter in one or two days.
[注] 表示“在一两天内”，应该说 in a day or two 或 in one or two days, 但不能说 in one day or two。类似的说法有 in one or two weeks/in a week or two (在一两周内); in one or two hours/in an hour or two (在一两个小时内)等。