For this story quest Ceara's instructions can be a little unclear on what she expects of you. Initially in this quest she outlines how to rank up skills with AP while asking you to pick a skill to train and rank up with AP.
The quest will prompt you to rank 'Combat Mastery' to Rank E, however, this is outdated text as Combat Mastery now requires a character to be Level 10 to Train and Rank up.
Instead Click on the 'Restoration' Tab in the Skill window and click on 'Battle Respiration' and train this skill. Once you have learned it, use AP to rank this skill to Rank E. This will allow you to meet the requirements to complete the quest and continue the main story!
The Quest will not automatically progress, so you will have to click on 'Story' For Ceara again and click on the quest 'Watch Out for Fomors' Once you do that she will rejoice in how easy Skill Training can be and you will receive completion rewards for this quest.
If you trained and ranked another skill instead of the Battle Respiration skill, no worries! A few Battle Quests will provide you with enough AP to train this skill once and complete the quest.