Dependent source while calculating Thevenin's voltage
You calculated the voltage across the 8 ohm resistor correctly, but then you assumed that the voltage at nodes a and b is the same as the voltage across the 8 ohm resistor...but they are not the same. There is a voltage across the dependent voltage source, which you must consider.

Find the voltage across the dependent source, then use KVL to find the voltage across a and b.