this does not answer the initial question at all. I'm having the same issue as the initial poster, and the answer given is about adding variants to a product already created without any variants, whereas the questions was about a product that was created with an initial set of variants, and then how to add another set of variants. The link about adding a variant does not answer it either, and does not even contain a video as claimed.
Thank you for reaching out for assistance in adding new variants to your existing products. It is a bit of a process to add variants once a product has been created and saved and the steps can be hard to find. I went through step by step and have written instructions and screenshots to help you out!
Open the product you wish to edit in your store admin under Products. Ensure that this product has been saved at least once and is not a brand new product you are creating. Near the bottom of the page you will see the VARIANTS section which will show any existing variants you currently have. Click MORE OPTIONS > EDIT OPTIONS to add a new variant option to the product. Please see image below:
Step 2: In the Edit Options pop up window, add a new variant option and fill in ONE style for this variant option. This is used to create the baseline for the new variant option. Click DONE once completed. Please see image below:
The reason you cannot create more than one option is because the system cannot tell from here if all existing variant options are included in the new variant option. An example of this is if you have a range of sizes and add a color option. Do all the sizes (sm, med, lg, xl) also have that color? They may not, which is why the system can only allow you to make the one new variant option.
Step 3: Save Changes. This step is important! You will not be able to add any additional new variant options until the product is saved at this point.
Step 4: To continue adding options for the new variant (Variant: Color in our example here), select all product options that will include the next variant option we are creating or select all products if all products will include it. Then click on MORE ACTIONS > Duplicate Variants ... In Another <Variant Name>. Please see image below:
Step 5: In the Create Duplicate Variants pop-up window, fill in the name of the new variant option (Color: Pink in our example), choose if you are duplicating any of the content for the product and then click the Duplicate button. You can leave the duplication section unchecked if you are unsure as most of these options can easily be filled in using the bulk editor later. Please see image below:
Step 6: Review the new variant option in the list and ensure that it looks correct. Once complete, make sure to save!
Please let me know if you are stuck anywhere or have any issues replicating this in your store admin!