A mix can show as out of stock even when it's listed for sale and has its own stock. The most common reason is that one of the bottles in the mix is out of stock.
Common causes
A bottle in the mix is out of stock. If any of the mix items is out of stock, the mix is out of stock too.
The mix has no stock. If stock tracking is on for the mix, check it has stock, counted the right way for its Count stock by setting. For example, with Number of bottles, a 12-bottle mix needs 12 stock for each mix.
Fix it
Go to Settings > Products > Mixes and click the mix's name.
Click Mix Items and note each bottle in the mix.
Go to Settings > Products > Bottles and check the Total Stock of each of those bottles.
Add stock to any bottle that has run out, or click Update Mix Items on the mix and swap it for a bottle you have.
If stock tracking is on for the mix, check the mix's own stock in its Stock section.