Kolbe’s electrolysis reaction involves 2 steps:
R-COO- ------> R. (alkyl free radical) + CO2 (decarboxylation of an alkanoic acid having n carbons to give an alkyl radical having n-1 carbons; min. being 1, that is CH3 radical)
then the radical dimerizes to an alkane,
2R. -------> R-R (an alkane having 2(n-1) carbons).
The lowest alkane formed itself has a minimum of 2 carbons (ethane onwards) for the reaction to occur. Since, Methane has only 1 carbon in total, this can’t be formed by Kolbe’s process.