Final answer:
To correct the sentence, add a comma after 'volcano' and change 'be' to 'he'.
Step-by-step explanation:
The subject of the question is English.
The sentence given is missing proper punctuation. To make it correct, we need to add or remove commas as necessary.
The corrected sentence is: After Gumball Watterson built a model of a volcano, he mixed baking soda and vinegar to make it erupt.
Learn more about Punctuation in English sentences