In one of our clients’ sites using WooCommerce, we got a popup message saying:
Sorry, no products matched your selection. Please choose a different combination. 
Even once we closed the error box, I couldn’t select one of the variations on a product. Argh!
I tried many suggestions I found online such as setting the default variation value and enabling ‘manage stock’ on a variation basis. When nothing worked, I thought I would have to painfully go through each of the WooCommerce template files in our theme and update each one. But then I took one last stab at it by adding a new product from scratch. And it worked!

The Problem

The problem had to do with how attributes were previously set up in WooCommerce.
For  a variable product, in the product’s attributes, the values for the Size attribute were set to: S | M | L | XL | 8 | 10 | 12 | 14 | 16  written out with separators.

The Solution

  1. Edit a Variable Product
  2. Go to Attributes, and remove the existing values remove1
  3. Add the attribute again (ex. Size) and click Select Allselect1
  4. Click Save Attributes (this is important for the next step)
  5. Go to Variations, and set the default size and re-assign each of the variations (the variations are probably still there but you need to re-map them).map1

And then, voila! No more error message!

It worked for us and I hope it works for you too!