I am facing the following issue,

Parent plan A (exclusive) has a few exclusive sub-plans (A1, A2, A3, A4, A5).

A field named X conditionally selects the plans to appear as follows,

X Field Option 1-> Parent plan + sub-plan A1
X Field Option 2-> Parent plan + sub-plans A2, A3, A4, A5

Restriction: As sub-plans are exclusive, only one can be set as default.

Since I don’t want the user to subscribe only to the Parent plan but Parent + one sub-plan according to the X field option, if I set A1 and A2 as default plans, it doesn’t work due to the above-mentioned restriction.

What I am trying to achieve here is when the user chooses Option 1, Plan A1 to be the default sub-plan and when the user chooses Option 2, Plan A2 to be the default sub-plan. This way, user will not be able to choose only the Parent plan because there will always be a sub-plan selected.

EDIT: Now I have noticed another point. Let's suppose that a user chooses a default sub-plan and he subscribes to it. In case the user wants to upgrade to another sub-plan (of the same Parent plan), there is no default plan in the available options because the user has already subscribed to it, thus giving the user the ability to choose only the Parent plan which is something that I don't want. Maybe my plan configuration is wrong?

EDIT 2: When a user wants to upgrade to another plan, the "Prorate remaining value of this plan when upgrading" is visible for all available sub-plans of the Parent plan the user is already subscribed to. Now, all other sub-plans that exist under different Parent plans (and are available to the user), do not show the "Prorate remaining value of this plan when upgrading" although it is selected in the settings of all plans.

The default select only applies to new subscriptions. This means the user must have no subscription at all (e.g. typically this is strictly just on registration). It won't apply to Upgrades, which are what all purchases are for registered users. They'll just need to be sure to select a subplan when selecting their subscription. Note selecting a child plan will automatically select its parent so they should only need to select their child plan and not have to do any further selections.

In the registration page, when user tries to sign up, I checked again and what happens is that

The plans (P) and (C1) are subplans of the same Parent with (P) being the default one.
Now in case you would wish to fill up all the mandatory fields you would notice that the message because of selecting the Parent plan only, would be "Error Your registration attempt has been blocked. Reason: Already registered.".

Sounds like none of your child plans are configured to allow upgrades under their Workflows tab.

Concerning the upgrade, you are correct that the user cannot select only the subplan as the Parent plan is also selected, so my mistake.

Not sure I understand what you're asking here. So you want prorating to work from Parent 1 Child 1 to Parent 2 Child 1? I'm not sure that'll work as prorating works within its parent plan. It would likely only prorate Parent 1 to Parent 2 and ignore the child plan prorating.

I will try to send images of what I mean for this.

Default select won't be applied if something is conditioned away. Your plans are all initially hidden so they're unselected. It won't re-select them when they're made visible.

Understood. Changed it and now none is default. Now if the user chooses only the Parent the purple notice will show up. I think it is ok this way.

That's CB AntiSpam blocking a duplicate registration from the same IP Address. That's working as expected and can be turned off within CB AntiSpam.

Tried turning off CB Antispam and all were as you said, thank you.

Not sure I understand what you're asking here. So you want prorating to work from Parent 1 Child 1 to Parent 2 Child 1? I'm not sure that'll work as prorating works within its parent plan. It would likely only prorate Parent 1 to Parent 2 and ignore the child plan prorating.

Please check image