I wanted to know -$ in credit means... do I have to pay back or I will get a refund (it says this ...Your account is in credit (-$75.75), please generate a statement)
For customer accounts that you have with a merchant, normally a positive balance in the account is what you owe the merchant. When you have a credit on the account or a negative balance, it usually means that it is an amount that the merchant owes you. This could be because you paid too much, or perhaps it is because the merchant is giving you a refund for some reason.
However, it is always a possibility that this particular merchant is not using the terminology in the conventional way. If you aren't sure, the best thing to do is to ask the merchant directly by contacting their customer service department.
Just judging from the screenshot you posted, it looks to me like they owe you some money. I suggest you click the "Statement" button; it looks like that will generate some type of statement that will instruct the merchant to send you a refund. But that is only a guess. Their customer service should be able to tell you what to do.
You have paid the vendor (school?), etc, $75.75 too much, for whatever reason. They can send you back this money, or it can apply against a future bill.
This is common when I pay my credit card in full, but also have a store return. The bill can show a credit, but it will quickly get used up by new purchases.