i got only one error please any one help me to resolve it. How to assign the Input pins to the caravel ? can any one guide me it is little Urgent (Simple and gate )

It seems you are doing the assignment the other way around. It should be

assign {a,b} = io_in;

There's also an extra pin (io_in has a width of 3 but you are connecting it to just 2 wires) that you are not using that might cause some problem/bug if you are not careful