Can't select simple product to associate with configurable product

Question

I have several configurable products with associated products. All have worked well and I am familiar with how to add both.

I have 2 products to add to one of my configurable products.

I added the first one by duplicating an existing item, and making necessary changes all the way through.

When I need to select it to add to the list of associated items, it's there but highlighted in red and not selectable.

I could not figure it out so I deleted the simple product and created from scratch. Same thing.

I can add it other configurable products, but not this one - UNLESS - I uncheck one of the existing ones. There are 5 there now. Is there a limit on how many simples can be associated with a configurable?

I cannot find this question on the exchange, so if you mark as duplicate, please also leave the URL to the answer.

Any thoughts? Magento 1.9x - Thanks for any input.

Answer 1

Usually this happens when the attributes used for the configurable product are not filled in or absent in the simple product. Or maybe the simple products that are already there have the same value for the attributes us as configurable.

Answer 2

Thanks to Marcel's comment it made me review everything. I had failed to add an attribute to the Selectable Options area. This item is the same as the one I had duplicated except one variant, and I needed to add that to the simple product.

Once I did - I was able to add it to my configurable.