You're not giving nearly enough information about your issue, "not detecting" could mean it's not detecting the syntax or the syntax is valid but it's not detecting you entering the region. In the case of the latter, that'd be your case. A few things to note
within the on region enter event, region at player != event-region. Region at player will grab the region of the player before they enter the region. event-region, obviously, returns the region that the player is about to enter (or already has entered depending on how you want to view it)
worldguard syntaxes do not work with skript if you are running fastasyncworldedit with worldguard, you must run worldedit and worldguard if you want these syntaxes to be recognised by skript
you can not directly compare regions with text, you must parse the region as a string/text first (see below)
regions, even after parsing them as strings, are formatted in the form "<region> in world <world>". Because of this, we use contains instead of a direct comparison (see below again)
A valid example of the on region enter event:
on region enter:
if "%event-region%" contains "<insert your region name here>":
cancel event
send "&cYou can not enter here!" to player
Hope this helps, Ofus.