When making elixirs and cures in your Lab you can switch the page of the list of the available items to produce. On the example of White Elixir:

White Elixir is the forth elixir, it is unlocked on level 6 and is located on the second page of the elixir list. To produce it in the Test Tube Tables:

1. tap on an empty Test Tube Table -> you will see the first three elixirs (blue, yellow and bubbly blue)

2. tap on the arrow button next to the area with the icons to turn the page -> you will notice there are more elixirs to produce on the second page

3. Produce White Elixir :-)