I am working on my assignment about sequence diagrams. my question is if can we include a creation or deletion message in a combined fragment
In short: yes. You can do that with no issue.
In more detail: try to stick to UML rules but don't get blocked by just looking into the law book. Of course it's best to not violate the rules set up by the UML gods. But like when you are standing a red cossing light at night where you can see and hear nobody else, then what would hinder you to cross the street anyway?
One common mistake seen so often is to forget putting join nodes in ADs which will make the whole network get stuck at a particlar action. Yes, wrong. But basically only to machines and most people can read the meaning witout any issue. Only the lawyers will pick that up as being wrong.
So to sum up: when you model something and you feel that it transports the message then it's okay. Even when being not 100% correct. Strive for correctness but don't let it block you from doing the job. If you raise discussion it ain't bad. Not at all! Models are meant to discuss about something. So if your model got a discussion started it's even better. You can always ask for advice and fix it later.