Minecraft – /loot giving “An unexpected error occurred trying to execute that command”

What is most likely happening is that you don't have an armor stand named ASCounter. The best way to test what is actually happening is by doing the command separate from the execute, so /scoreboard players add @e[type=ArmorStand,name=ASCounter] counter 1 Then it will tell what is happening wrong.

You need to add more escape characters to deeper-nested quotation marks. You open the Text1 tag with a ", thus the very next " character closes it. You open the value tag for the click event with \", meaning the next \" you use will close it, which occurs in your nested /give command's NBT data (specifically just after the Name tag). That it what's causing your issue.

The formula to determine the number of backslashes required is: 2n+1, where n is the number of current backslashes. For the nested NBT data, you need 3 backslashes per quotation mark (\\\").

Fixed command:

/give @p sign 1 0 {BlockEntityTag:{Text1:"{\"text\":\"[Sell]\",\"color\":\"dark_purple\",\"bold\":true,\"clickEvent\":{\"action\":\"run_command\",\"value\":\"clear @p[name=rainbowlack,r=3] cactus 0 2304\"}}",Text2:"{\"text\":\"2304 Cacti\",\"color\":\"red\",\"clickEvent\":{\"action\":\"run_command\",\"value\":\"give @p[name=rainbowlack,r=3] minecraft:cookie 18 0 {display:{Name:\\\"$100 Cookie\\\",Lore:[$100]}}\"}}",Text3:"{\"text\":\"iIiIiIi\",\"color\":\"light_purple\",\"strikethrough\":true,\"obfuscated\":true}",Text4:"{\"text\":\"[Sell]\",\"color\":\"dark_purple\",\"bold\":true}"},display:{Name:"Custom Sign"}}