I often see would-be bloggers and writers held back by the fear that they lack expertise. They believe they don't know enough about a subject to write about it.
This is exactly backwards. When I started my food blog Casual Kitchen, I knew nothing. Just a few ideas about how to cook easy, inexpensive meals at home. And I had no idea how staggeringly obvious these ideas were until much later.
That was the full extent of my expertise when I started my own food blog.
Over the next few years however, I began writing about more aspects of food. I slowly and tentatively pushed into new areas. I started discussing food philosophy, addressing ethical issues surrounding food, and I even wrote about how to invest in the food industry. Some of these posts were failures, but some turned out tolerably well. And of course the entire point is the act of writing about things I didn't know taught me what I needed to know.
I am now a minor authority in the world of food blogs because I wrote all of that content. I stumbled through it ignorantly. And now, ironically, I more or less know what I'm talking about.
"Not knowing" is the reason why you should write, not a reason you shouldn't.