PowerPoint is typing only in capital

Before anyone can offer pertinent suggestions they would be wise to collect a bit more detailed information about the issue, such as:

• From PowerPoint> About PowerPoint in the main menu, what is indicated as the present Version & License type of your Office 2016 installation?
• Does this occur in 1 specific presentation file, or does it occur in all files, including newly created files?
• Within a specific presentation, does it happen with all placeholders or just specific ones?
• What other details can you provide?

One possibility is that you've inadvertently toggled Change Case, possibly by using the keyboard shortcut Shift+F3  (fn+Shift+F3 on a laptop or Apple Wireless keyboard). Can you convert the case by either of these methods?... Select the text, then;

1. Use that keystroke to toggle through the case options, or
2. Use the Change Case button in the Font group on the Home tab, or
3. By going to Format> Font to clear the check for All Caps

Some templates are set up intentionally to convert to all upper case. Do you see this happening when you open a new, blank template?

To add to what Bob said, if this is happening only in a single presentation, make sure that the font you're using actually has lowercase letters. Some fonts are all caps.

If your Caps Lock isn’t on and it is still in all caps, restart PowerPoint. If that doesn’t work, restart your computer.

A few possibilities in addition to Bob's suggestions (and frankly, I'd ignore the other replies, at least for now) (except for Suzanne's):

Does the problem continue after you've powered down and restarted your Mac?

Does this happen only in PowerPoint and not in, say Word, Excel or other Mac programs?

Does this happen only in one particular presentation or group of presentations?  What happens if you start PowerPoint, choose the default blank presentation and add some text?  Still all CAPS?

What font is the text set in?

Your answers to these questions will help us zero in on or eliminate a couple of possibilities:

• Stuck CAPS LOCK key
• Slide master or Layout set to produce all capital letters
• Using a font that includes only uppercase (CAPS) letters