Transfer your library to another computer. When you sync it will be looking for that library.
Moving a library: Quick answer if you use iTunes' default preferences settings: Copy the entire iTunes folder (and in doing so all its sub-folders and files) intact to the other drive, hold down the option/alt key while starting iTunes, and when requested to select a library select the iTunes Library.itl file in the copied folder.
When this is to a new computer put the copied iTunes folder in the default location of Macintosh HD > Users > *User Name* > Music (or Home > Music) then you don't even need to start with the option key held down, iTunes will automatically look for it there. (Make sure there isn't anything already in the iTunes folder there that you want to keep since you will be replacing it with the one you are moving.) Remember to authorize the computer in iTunes to your Apple ID if you use this to sync to Apple mobile devices or play DRM protected media (e.g., movies).
Ideally you should then restore the computer to like-new condition.
Linc Davis Feb. 2016 post on restoring computer to new condition - https://discussions.apple.com/thread/7447000?answerId=29726244022#29726244022