I did a two way ANOVA and proceeded for a Tukey Post Hoc Test. I got this warning that Post hoc tests are not performed for Medium Concentration because error term has zero degrees of freedom.
You have a saturated model, that means the model you are analyzing includes all factors with all interactions and there are no repetitions on your responses (for each test you only have one response).
You have to identify factors and/or interactions with low contributions and remove them from your model. By doing that, you will get an error (which is constituted of all factors and/or interactions you removed).