Here's why:
* Missing conjunction: The sentence needs a conjunction (like "if" or "unless") to connect the two clauses.
* Incorrect verb tense: The first clause uses the future tense ("will put on weight"), while the second clause uses the present tense ("get enough exercise").
Here are some ways to fix the sentence:
* "You will put on weight if you don't get enough exercise."
* "You will put on weight unless you get enough exercise."
* "You will put on weight if you aren't getting enough exercise."