Sass Error: '90var(--num) is not a number' - How to Fix

This error occurs because the variable '--num' is not being recognized as a number by the Sass compiler.

To fix this, you need to ensure that the variable '--num' is defined with a numeric value.

For example, you could define it like this:

:root {
  --num: 10;
}

Then, your SCSS code would work as expected:

@for $val from 1 to var(--num) {
  // loop code here
}