Is this because I installed macOS Sierra from Yosemite a couple of months ago, it's on my B Mac so don't use it so much.
From the screenshot I am assuming you're trying to save an attachment directly from Mail.
If so, there is a known issue that a corrupted Mail index causes this odd 31 character file size limit when saving from Apple Mail. The best way to correct this is to follow Apple's instructions for rebuilding your Spotlight index.
If my assumption is wrong (re the Mail app), let me know and I'll update my answer with broader solutions beyond just the Mail app issue.
Per this Macworld article, you could try the following and see if they help.
- You’re trying to save out of Apple’s Mail app and it has a corrupted index. Many users across many forums and years report being told the character limit for a volume is 31 characters, then re-indexing Mail, and the problem disappears. Go figure! But if it works, it works. The simplest way to fix Mail is to re-do your whole Spotlight index per Apple’s instructions. (There are outdated instructions on Macworld and elsewhere for deleting Mail’s index by itself, but these seem unreliable in El Capitan.)
- If you’re using a network or external volume that’s not formatted for Mac OS X (as HFS+), you may be hitting a limit for that other format.
- Some software was never properly updated to move from a previous maximum 31-character limit many, many years ago—this dates back to Mac OS 9! But for compatibility’s sake, some software throttled filename length. Apparently, some apps still adhere to this.
- Your full Unix path may be too long. There’s a soft limit of 1024 characters for the entire path from root to your file (like
/Users/glenn/Documents/Hope Springs Eternal/Smashmouth/Ringbearer/Oodles.txt/). Some users report being able to create paths longer than that, but then experiencing problems. You may need to repair your disk. The best way is to boot into OS X Recovery, run Disk Utility, and run Repair Disk on your startup volume. Restart your Mac, then hold down Command-R until you get the recovery screen. Launch Disk Utility, select the startup volume, and click Repair Disk; do the same for the partition in that volume. Then Restart.