Hello! This could be for a few reasons I think, but the most likely one is that you need to type in a Summary (at least, and ideally also a Description) for the commit.
So, here's my commit button blanked out 'cause I have no Summary:
But as soon as I start to type a Summary, I can now commit:
That's likely it, but if you need any further help, let us know. Providing some screenshots will help show what you're seeing as well, so if applicable please upload one to help explain.