First, welcome to the game. There's a lot of differences from FFXI, especially in gameplay mechanics.

The game will introduce a gameplay-changing ability at levels 15, 30, and 50, following completion of a quest. The introduction of these abilities create the greatest changes to your skills and rotations. Generally though, after level 30, you will have a solid understanding of how the class works.

If you are asking if there is any content in the game you cannot handle solo, it is really only instanced content like dungeons, trials, and some FATEs with the equivalent of world bosses. Your first dungeons unlock at level 15 and are very rudimentary. They do get much more difficult starting from lvl 32 to 35, and there can be a few significant hurdles at lvl50 as well (Hard Mode, Coil, and Extreme Mode). Generally though, you will not need a static party outside of the dungeons to complete the story and run quests before level 50.