"couldn't enter" suggests that there was some external barrier or obstacle that prevented you from entering the campus. This could be due to various reasons such as a locked gate, a security guard blocking your way, or a crowd of people obstructing your path.
On the other hand, "didn't enter" simply means that you did not enter the campus without any specific reason or explanation given. It doesn't necessarily imply any external obstacle preventing you from entering.
In the context of your essay, if there was indeed an external barrier or obstacle that prevented you from entering the campus, then "couldn't enter" would be more appropriate. However, if there was no specific reason why you didn't enter the campus earlier, then "didn't enter" would be a better choice.