presented with that problem, the first thing I'd do is copy your text and (hopefully) paste it into an email to myself and send it.
Then I'd follow the (in)famous tech support offered up on the British comedy, 'The IT Crowd', "Have you tried turning it off and turning it on again?"
My smartphones, my MS computers (laptop and desktop and my tablets go wonky on me if they stay on too long. Only my Chromebook seems to avoid this foible.
Try tapping outside of the edit area (like on the gray "header" area of a post. That should free up the focus from the edit area and let you scroll down and be able to tap on the "Post Message" button.
What you describe usually happens when the focus is "stuck" (often because of a text selection or copy/paste operation, even if inadvertent) on a text edit field. Tapping outside of the text edit field will remove the focus from there, and free you up.
I have an android and the exact same issue, but test my work-around on a new page to avoid losing you post.
1) Type a few characters in the message field.
2) Push the "Go Back" button on your phone to get to the main Message List. (I have to push it two times, once to "go back" to the message where I had entered no characters, and again to get to the message list.) The button is on the lower most right-hand corner of my phone.
You should noe be on the message list.
3) In Chrome, click on the three little dots in the upper right-hand corner, and click the "go forward" arrow. I have to do this twice, too.
These steps take me back to the few characters I had entered, and the command buttons for the message field are correctly placed and useable.
Short version: back, back, forward, forward. That simple.
Please let me know if my sexy-dexy "tech lingo" ;) was unlcear (not tech enough - no guru here!)