Hi. I will try to help you with this.
There are two possibilities.
First, have you actually created a project? If you have clips drag/copied from an event or photos drag/copied from the photo browser, you have to create a project and save your work as such before you will be able to share it, that is, before the share tab will appear ungrayed.
If you have created a project and the tab is still grayed, this is a known bug. Try renaming the project and saving it afresh. This should ungray the share tab.
Thank you for your cooperation.
I can't tell you without seeing the files. The fact is that, if you have published a project, and you are unable to share, this is a known bug with Mac. Apple Care suggests creating a new library.(Have you upgraded OS versions recently?) If that fails, the only possibility is that one or more of your files are corrupted, which could be for any number of reasons.