while strcmp(condition,"yes") == 1
year = input (' Enter the year : ');
elseif mod(year,100) == 0
Okay, that else matches the elseif which is associated with the if mod(year,4) so it is fine.
But that else does not match anything. To use else, the previous control statement has to be if or elseif, not else
condition = input("Would you like to repeat? (yes/no)\n", 's') == "yes";
I would also point put that if a year is NOT divisible by 4 (as needed to reach the elseif) then it cannot be divisible by 100.
Perhaps you wanted an else or elseif there?