The project allows adjusting the operation of the fan, depending on the temperature. To guarantee the satisfactory operation of the fan in any situation, its supply must not fall below the starting voltage. Its value is equal to the supply voltage of 12V minus the voltage T3-R6-R7.

With the values given in the diagram, the fan supply will be at least 7V. If the fan does not start at 25C degrees, change the value of RV2. When the temperature reaches 35C degrees, the circuit must start operating, which is indicated by increasing the fan speed or its supply voltage. If this does not happen, change the value of RV1. Thus, as the temperature increases, the fan speed will increase. You may want to change those 2n3904 transistors with 2n2222.

If you really like what I present, you know what to do.