Why get an error message while installing Office Program in my PC

Can someone help me, please?
My problem is that, I tried to install an office program in my PC, but an error message kept on popping out. It says “The installation source for the product is not available".
I don’t know why it is like that when in fact the DVD is inserted in the disk drive.
Hoping someone could give me a response to this.
Thank you.
