I give up, where can I start farming Red Crafter's script?

I've talked o the NPC in Mor Dhona, In Idyllshire and went to the appraiser in Gridania.

I've set the item to Collectable mode, but the Collectable Appraiser seemed unable to accept the craft. Am I missing something?

Are you giving them the items they ask for in the Rowena's Splendors tab of your Timers window?
Are the quality ratings high enough? They won't take them if they're too low.

^ There is a min collectability required for the NPC to accept the item, to increase collectability you need to increase the quality of the item and ofc you need to turn in the item the NPC requests not any other ones

Best way is if you have access to custom deliveries from Zhloe or M'Naggo. Those give tons of scrips and are dirt cheap to create the items for (since its vendor materials).

For anything else, you need to look at the collectable list under Timers > Rowena's House of Splendors to see what it is they want, at what rating. That will also tell you how many scrips its worth.

And just putting this out there, but you did the quest to unlock collectable turn ins, right? The quest actually starts in Ishgard, you get something like a letter of introduction to give to Rowena's collectable team, and then you can start turning things in. Beyond that, they change what they want daily(the mid-afternoon EST reset - they share it with the GC turn ins) and you need to hit the minimum quality level for them to accept it. Best of luck!