How do I set a glitched text on a sing in Minecraft?
You have used both the wrong name and Syntax in your command, this would be your desired command:
/setblock [x] [y] [z] oak_sign[rotation=0]{Text1:"{\"text\":\"This text will not be seen\",\"obfuscated\":true}"}
Remember to change rotation to change where the sign is facing.
You could also use the data command to change what a sign says:
/data merge block [x] [y] [z] {Text1:"{\"text\":\"This text will not be seen\",\"obfuscated\":true}"}