Answer:
Change the comma to a period.
Step-by-step explanation:
"It was a beautiful day, there wasn't a cloud in the sky." Has two complete sentences mashed together with a comma. The correct way to add two complete sentences is with a coordinating conjunction. A coordinating conjunction uses a comma followed by a conjunction like and, or, or but. This sentence has a comma but NO conjunction. It also doesn't have a semicolon, the other way you can add to complete sentences together.
Thus, it must be split into two with a period. This is the last answer, "It was a beautiful day. There wasn't a cloud in the sky."
I hope this helps you!