My Druid is lvl 49, but unable to select the level 30 talent Ursol’s Vortex.
Tested on a similar level Druid (48) and can confirm that you’re unable to pick Ursol’s Vortex for some reason.
Tiger Dash and Wild Charge don’t encounter this error.
When attempting to pick the Talent (regardless of Tome use or Rested Zone), you get the error message “Unable to learn talent”.
Guardian Druids above a certain level (tested at 90 and 111 respectively after) don’t have this issue.
All I can think of is an incorrect Talent Level flag, though attempting to select a talent you can’t use yet doesn’t bring up this error message, or any message at all.
Hmm, in the tooltip on wowhead it appears that the talent is flagged as “Requires Restoration”, instead of “Requires Guardian”.
There are two versions of the Talent that you’re looking at, and WoWhead has pages for both.