A P6 error code occurs when the pressure of the system goes below a certain value. This is caused when the pressure of the system is too low for the unit to operate.
A quick test to perform in this case, is to set the unit to the maximum heating temperature, run it for 15 minutes and check the larger pipe coming out of the outdoor unit. It should have a temperature of around 105 F which is too hot to touch. If the pipe isn't hot, then the system is low on refrigerant and there could be a possible leak somewhere.
Another thing to note, is the pressure of the system while in heating mode should be around 400 psi and in cooling mode a 100 psi.
You can also review the attached PDF for a more through diagnoses of the issue. Please refer to the other attachments for the specific tests mentioned in the error code procedure.