The less than (<=) operator needs two operands. You only provided one.
Your code correctly is as below:
#include <stdio.h>
int main ()
{
float tax = 0, income;
printf ("Enter your income:\n");
scanf ("%f", &income);
if (income >= 250000 && income <=500000)
{
tax = tax + 0.05 * (income - 250000);
}
if (income >= 500000 && income <=1000000)
{
tax = tax + 0.20 * (income - 500000);
}
if (income >= 1000000)
{
tax = tax + 0.30 * (income - 1000000);
}
printf ("\n your net income tax to be paid by 26th of this month id %f", tax);
return 0;
}