Because of the required perform level, this feat cannot actually be taken until character level 22.
So in Layonara rules that means that if you want to take Lasting inspiration as soon as possible you should not multiclass because you need to get those straight 5 levels of new class unless you multiclass with Mistone Alliance Scout (if it really is same as Harper scout) and take Lasting inspiration feat in level 24 (yup, using level 24 normal feat)
Off topic, but now there are no more Mistone Alliance Scouts, but Are called now Agents of the realm . *put the curtain back to topic *
Lore pages need urgent update then!
Duelists have perform as a class skill, so the only thing holding that back would be if Lasting Inspiration is on duelist's feat list or not.
And although you can get the perform ranks as a class skill, Lasting Inspiration should not show up as a standard choice at level 21 or 24 on the duelist or bard feat list.
2. If you decided to take level 21-25 as a duelist, you could take 1 level in bard at 26 and get LI then...
NOTE About Feat Prerequisites: BA must same as needed feat prerequisite before starting to level up to get feat. Skill points work differently and added skill points count when leveling up and adding them just before taking the feat. Reason to this might be that BA upgrades only after leveling up..