Please show a screen shot with the error and your new value. Please do NOT try to email it, but return to the forum and use the insert image icon in your reply.
This is definitely not supposed to happen. I just tried with Acrobat DC on El Capitan (don't have Acrobat installed on my Sierra machine), and it definitely works here.
Any time I run into a weird problem like this that does not have a good explanation, the first thing I do is to reboot the computer. We are running Mac OS, so that very likely means the same for you as it does for me: We don't reboot on a regular basis. Sometimes a reboot is all it takes to restore normal functionality. So, I would reboot and after the reboot only start Adobe Acrobat and no other application (of anything comes up, I would exit those applications). Then, I would turn off all the other optimization categories besides "Images". Then, I would try to use a different downsampling algorithm, including "Off". Does anything allow you to enter a number? If that all fails, I think it's time to get Adobe's support involved.
I'm running into the same issue. I can't remember when it started but now affects my Acro Pro 11 and Acro Pro DC. No matter the value or settings entered into Save as Optimized, it says "invalid numeric value." My previously saved presets appear to work (but I can't adjust them.)
MacOS Sierra 10.12.1
Acrobat DC 2017.009.20044
Same for me.
Same for 3 other Sierra users here at work.
Acrobat Pro 11.0.19
Doesn't matter what number I enter or which field I start in - can't create and customized optimization.
Works fine in same version of Acrobat in Yosemite, with the same PDF.
I wonder: what are you national language settings, and what is he language of Acrobat?