Stereoisomers of 1,3,4,6-tetramethylcyclohex-1,4-diene
Your compound, 1,3,4,6-tetramethylcyclohex-1,4-diene, does not have line symmetry (plane of symmetry), and hence $\ce{C}$3 and $\ce{C}$6 are chiral centers. Thus, maximum possible stereoisomers are $2^2 = 4$. However, there is a symmetry element in the structure, which is order 2 ($180^\circ$) rotational symmetry (also called point symmetry). Therefore, one stereoisomer, $(3S,6R)$ would be superimposable with it's mirror image $(3R,6S)$, thus it is a meso-isomer:

To understand this, I put &quot;$\bullet$&quot; symbol to indicate relevant hydrogen is above the plane of paper. And, &quot;$\bf{.}$&quot; symbol to indicate relevant hydrogen is below the plane of paper. Thus, first two structures have both 3,6-dimethyl groups below the plane of the paper while the third structure has 3-methyl group below and 6-methyl group above the plane of the paper. Accordingly, all chiral centers have CIP nomenclature as indicated under each structure:

The first structure is $(3S,6S)$ and the second structure is $(3R,6R)$, thus they are non-superimposable mirror images as indicated by the mirror. Therefore, they are enantiomers. To make all the groups to coincide, you may need to rotate the second structure $180^\circ$ horizontally along the axis going through $\ce{C}$3 and $\ce{C}$6. However, after the rotation, the two methyl groups at $\ce{C}$3 and $\ce{C}$6 will be now above the plane. Therefore, these two structures cannot be superimposable.
On the other hand, the third structure is $(3S,6R)$ ($\ce{C}$3 methyl is below the plane and $\ce{C}$6 methyl is above the plane). To the mirror image of this molecule, if you follow the same operations as previous steep and do another $180^\circ$ rotation on the plane of the paper (or vise versa), you'd get the same compound.  That means, the third compound is not optically active and called meso-isomer. This can be found easily by looking at the two chiral centers: They are mirror images of each other, $(3S)$ and $(6R)$ or $(3R)$ and $(6S)$.

Therefore, your compound, 1,3,4,6-tetramethylcyclohex-1,4-diene, has three stereoisomers, but only two of them are optically active.