One common reason that your Google Plus business page may not be capable of receiving reviews (there is no “write a review” button showing up on your page) is that it may not be set up properly. If you are sure that you have a business page and you notice that 1) it is not verified (does not say “verified” when you log into the page as Manager 2) has no visible reviews or a place that the public can click to receive reviews, then it might have been set up as a Brand Page. Brand pages are not eligible to receive reviews and they do not get verified like a local business page gets verified. Only local pages (Services and Storefronts) can get Reviews.
Can you convert a brand page over and receive reviews? If you are eligible to receive status as a local business, then Google allows you to essentially merge your Brand page with a new local page that you create. You will then need to connect your existing Google Plus page with your Google Maps page. It can be confusing; however, here is some information below. And, if you really get stuck then you can always contact Google directly. If you get in touch during normal hours, very frequently, you can get them on the phone and walk you through the fix directly.
Here are some links to help you below. Note that businesses can now (since 2014) “upgrade” their Brand page (or any non-local page) to a local page.