Recall that geometric isomers are two or more compounds that differ from each other in the way in which they are arranged in space (spacial orientation), i.e. the way the groups are arranged around a double bond, a ring structure or any other rigid structure.
In a linear structure, the bond angle is 180º and so no matter how you rotate the molecule, you cannot find a geometric isomer. And the same holds for the tetrahedral structure where the bond angles are 109.5º. However, in alkenes, there are geometric isomers, and these are sp2 hybridized, making them trigonal planar. So, while you cannot have geometric isomers for linear and tetrahedral structures, you can have them for trigonal planar structures.