I don't think it will cancel the work but sitting with your spine straight will help you feel your chakras better and pronounce the words better. The choice is yours.
That's not a serious effort. You either do something or you don't. If you are too tired, just don't do it. There's no point in doing a shit ritual and then feeling shit about yourself that you did a poor job.
The more you put in, the more you get out. I see no scenario in which you can ever justify doing any ritual like that. Would you do a ritual to praise a God slumped over in your bed? Why would you do an RTR any differently?
Something a lot of people don't understand is that you don't "get" energy to do something; you generate it. Kind of like you can wake up full of energy and walk into math class and be instantly depressed. Or you can be completely "exhausted" and something beautiful happens to you and you feel your whole body vibrating with energy for the rest of the day.
I would say, sit down with intention and just begin the ritual anyway, despite of how you feel. Don't sit there and try to calculate how long it's going to take or if you think you "have" enough energy to do it; just start without any of that. After a few vibrations you will feel better and you will finish the ritual feeling energized, rather than depleted.