Error message ITMS-9000

Hello, I keep getting a message that says 'referenced resource missing in the package.' I have about 20 or more of these, and each of them are connected with a web link that I've created in the epub. Seems there is some code missing or wrong. I don't know code, and would appreciate support in fixing this so that the link takes the reader to the web page when they are reading the epub. Most importantly, that I can deliver the epub to ibooks. thank you