The entire last line of this does literally nothing right? So they can't cast spells "As their action while they have priority"... but they can cast spells as long as it's not their action? Or while they don't have priority?

What's it even supposed to be doing? I presume that ~ is the card's name. So the last ability is just supposed to stop players from casting spells once the ability is on the stack? I get that this is a pet hate piece that you really want to use to stop people casting spells during your turn, but there's very little that they'd want to cast after their main phase, but before you choose what type of spell to block.

In the end, isn't this just Defense Grid but with a lot more words? And it's now an enchantment and only blocks one particular type of spell and it hits your spells. Wow, what a crazy design.

Not to mention "the active player"? How about 'that player'. Teferi's Realm.

About the wording of the first ability, the intention of doing it this way is that if the shared team turns option is in use, the entire team makes a single decision which applies for that turn, instead of each player doing so individually.

Maybe the wording for the second ability is also unclear. It is supposed to allow spells to be cast as a part of another effect which casts a spell as a part of that effect, but not to allow casting spells in the usual way, until the first ability resolves.