Why won't this mod work?

[groundFungus]

@mil24

if (ledState == LOW) {
      ledState = HIGH;
      long interval = 100;  // local to this if block.  Not visible outside the if

Since you have the long data type specifier in front of the interval there, that is a local variable so is only visible inside of the if block. It is a totally different variable that the one declared above setup() and used in the millis() timer.

A link to a tutorial for what @anon73444976 mentions.