Creating Variable Products
After you enter the attribute name and list of values, you need to select the "Used for variations" checkbox. This is in the screenshot in the instructions but I don't believe it's mentioned in the text. Also, after creating the attribute, you have to save the product to get the variation options to display.
Once the variation options are visible, it's generally best to select the option "Create variations from all attributes" in the Add Variation menu. After clicking the Go button, all possible combinations of attribute values will be created as variations. If, for example, there are two attributes (say size and flavor), each of which has three values, six variations will be created. The variations might be something like Small/Mild, Small/Medium, Small/Sharp, Medium/Mild, Medium/Medium, and so on.
Finally, and this is the crucial step that is easily missed, you have to set a price for each variation. The price field that is available at the top of the page for simple products disappears when the product is switched to a variable product, and the product will display as out of stock and unavailable when no price is set.
The way to fix this for variable products is to supply a price for any variations that you want to offer for sale. In most cases, this will be all variations. If the price varies among the different variations, as it might by size, you can set the price for each variation by clicking the tip-down arrow to the right of the variation and enter a value in the price field.
Click Save Variations below the variations table or Save Product at the bottom of the page to finalize the changes.
If all variations have the same price, such as in the case of flavor variations, select "Set regular prices" in the variation menu and then click Go to be prompted for a price to be used for all variations.
Click Save Product to finalize the changes in price. If the price ever changes, regular prices can be changed by repeating the process.