Why can't I see some Value Rules that I created in Google Ads on the Optimize Value Rules - VBB interface?

There can be 2 possible reasons for this:

We do not show any existing value rules that have multiple segments OR-ed in either the primary condition or the secondary condition (A warning is shown when such exclusion is done).

For example, if you have a value rule where the primary condition is 'Location is US or India' and the secondary condition is 'Device is Mobile', we will not show such a value rule in our interface as it contains two location segments in the primary condition. Our value adjustment suggestions are only for a given combination of 1 primary condition segment and 1 secondary condition segment.
The existing value rule that you've set up contains an audience category apart from Affinity, In-market, and life events. We support only these audience categories due to the limitations of the Google Ads APIs. This means you will not see any rules containing audiences apart from these as well as you will not be able to create value rules for audiences apart from these.

Why do I see this warning?

This warning just notifies you that you have scored less than 30% of cards for a given condition type, say Device Category. In this case, we suggest you score more cards that condition type in Score Traffic Segments so that we have data to give you the best suggestions possible. That being said, we suggest conversion value rules if you've scored any of the segments relevant to the segments selected in your condition types for the value rules.

Why am I seeing the message "To change the primary and/or secondary conditions, you'll need to delete all the existing value rules and create new ones"?

This appears when you have already set up conversion value rule(s) for particular segments in your account. As stated in this Google support doc, all value rules in your account must use the same type of primary and (optional) secondary conditions.

For example, if your primary condition is "Location" and your secondary condition is "Device" for value rule #1, all other value rules must have a "Location" primary condition. If your additional value rules also have a secondary condition, it must be "Device".

If you need to change the type of conditions you're using in your value rules, you'll need to delete all value rules and create new rules with consistent conditions. To do this, please visit the 'Value rules' section in the Google interface.

Which Audience types are supported in the Optimize Value Rules - VBB tool?

The tool only supports the audience criteria type “user interests” and "user lists". For this, the following sections are supported:

This will be updated as and when Google API begins to support more audience options.