Why are cis and trans isomers of succinic acid impossible?

Due to the absence of double bond in Succinic Acid.

Explanation:

cis-trans Isomerism also known as Geometrical Isomerism is a type of isomerism in which isomers have same molecular formula and structural formula but differ in respect of arrangement of groups or atoms about the double bond.

Therefore, a double bond is the main requirement for such isomerism process.

Example:

cis and trans Butene. In cis-Butene both methyl groups are located on the same side. While, in trans-Butene both methyl groups are in opposite directions (Figure red).

Result:

As shown in figure, succinic acid (blue) is lacking a double bond. Therefore, it fails to show cis-trans isomerism.