How come islanders don't know to create titan serum even though the Reiss family have them? We know that Rod had titan serum to convert someone into a pure titan in the episode where he himself is converted to a giant caterpillar-like titan. But in chapter 107 visitor, it is implied that Islanders don't have the knowledge to make titan serum and they only have those which were gifted to them by visitors. Clearly, someone on the island knows how to make them. Then why can't they find that someone probably in the government who had ties to Rod? Since they overthrow his government, they should know about all his secret technology.
This might be the knowledge that only the Fritz family knew about, which Grisha Jager killed in the first place. or maybe, only the wielder of the founding titan from a royal bloodline knows how to make it. As you might know, every royal who inherits the founding titan also inherits the previous wielder's memories and king Fritz's will of peace.