To set a manual Critical Power, you need to have a Stryd pod that has been connected to your account and that you have disabled Auto-Calculated Critical Power.

Once you've successfully paired a Stryd pod to your Stryd account, then you will be able to set a Manual CP by going to Settings->Critical Power->Selecting Recalculate

If you receive a server error when trying to re-enable Auto-Calculated Critical Power it's because you do not have a Stryd pod that has been connected to your account and will be unable to re-enable until you've paired your Stryd pod to that account.