In your example, the difference in nuance is based on what the subject of the sentence is. Technically, the subject of "마세요" is "you", but the subject of "안 돼요" is "something"

I'll show you with direct translation

그러지 마세요. = You shouldn't do this.
이러면 안 돼요. = If you do, something won't work.

"안 돼요" is "안"(not) plus "돼요". "돼요" has many meanings, but the closest one in this example is literally "work".

So, "이러면 안 돼요." is a more indirect way of saying "You shouldn't do that" by stating the circumstances of the situation rather than speaking directly to the other person, and can have the effect of being polite. These expressions that omit the subject(something) and use "돼요" are so common in Korean that it's hard to recognize that a subject has been omitted.

In your example, the difference in nuance is based on what the subject of the sentence is. Technically, the subject of "마세요" is "you", but the subject of "안 돼요" is "something"

I'll show you with direct translation

그러지 마세요. = You shouldn't do this.
이러면 안 돼요. = If you do, something won't work.

"안 돼요" is "안"(not) plus "돼요". "돼요" has many meanings, but the closest one in this example is literally "work".

So, "이러면 안 돼요." is a more indirect way of saying "You shouldn't do that" by stating the circumstances of the situation rather than speaking directly to the other person, and can have the effect of being polite.

The expression "돼요" with the subject omitted is used so frequently that it's pointless to specify that the subject is omitted, but it's worth knowing its origins to understand the subtle differences in meaning.