In my experience claiming the throne when a noble is expected to succeed will make it so that noble is of your dynasty (instead of whatever before the claim). That means you will be one step closer to a PU, but you won't be there yet.
In this case, he already has the same Dynasty. Claiming the Throne should give him a CB to enforce it. But I believe that's being blocked by Castile having its own PU minors.
I've seen this case specifically, and with the same countries.
Spain had "A Noble from House Lancaster will succeed", with Lancaster being my dynasty. It said that for many years, then suddenly it changed to "PU with England." I was mystified, so I went back through many years of save games (wasn't Ironman, thankfully!) and realised that it changed at the moment that Spain integrated their PU minor, Naples.
It tries to block you from taking into PU a nation that itself leads a PU.
However, this can still happen forcibly, through war or event. If it does happen, such that e.g. England gets a forceful PU with Castile, who are PU-master of Naples, then England ends up as PU-master of both Castile and Naples.
This is very nice when you DOW Spain on the Claim Throne CB, and the act of DOWing them changes "PU with England" to "PU with Portugal" (who also has same dynasty.) Then, during the war, Spain's monarch dies. So they fall into PU with Portugal. But you're still at war with them. Spain drops out of the war, Portugal becomes War Leader, and the game is set to apply "Force PU with.." to the War Leader, not a specific target. So you fight Portugal, you get 84% WS, you force the PU, and that PU's Portugal, not Spain - even though you Portugal has an heir and you never (and couldn't) claimed their throne! Then, because Portugal is PU master of Spain, you end up with a PU of both Portugal, and Spain! When you only claimed the throne of Spain
Beautiful when it happens
(Note it can happen in even more ridiculous scenarios - e.g. DOW Spain on Claim Throne CB. They call in ally Austria. Spain drops out of the war for some reason, and Austria becomes WL. Now when you get 84% WS, you get a PU with Austria! despite them having a healthy monarch, healthy heir, different dynasty, and you not even having an RM with them.

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