The F6 error code for this model indicates a drive motor tachometer error. The control board is unable to properly detect the motor speed. If you have a volt/ohm meter I can give you a few checks that will help you diagnose the problem.
Always unplug or de-energize electrical equipment before removing covers or attempting service. Be careful and wear appropriate hand protection when working around sharp metal parts.
You will need to gain access to the MCU (Motor Control Unit), and ohm the motor windings and the tachometer (see images below). The image that shows the MCU connection should be as it is in your washer, there should be 5 black wires and 1 green wire. The black wires are as follows, pin 1 on top and pin 5 on the bottom (see image). Set your meter to read resistance and check pin 1 to pin 2, pin 1 to pin 3, and pin 2 to pin 3, these should all read approximately 6 ohms. Next check pin 4 to pin 5, it should read approximately 115 to 125 ohms. If any of these readings are not very close to the values I have given you, check continuity on the wires from the MCU to the drive motor. If the wires check good, the drive motor is bad and will need to be replaced. If all readings are close to the values I gave and there is not an issue with the wires the MCU will need to be replaced.
You did not give me the model number; therefore I am unable to help you with any part numbers. If you have your model number I am including a link that you may input your model number and see a parts breakdown, please click here;
Sears Parts Direct
If you require further assistance please resubmit your question with details of the checks you have made. I will be looking forward to helping you again in the near future.