Fail with error 'Crowdsale: rate is 0'

as a beginner in solidity, I am writing to you to get some information about the error message which is the title of this topic "Fail with error 'Crowdsale: rate is 0'".

I am trying to set up a crowdsale with a token with 8 decimal. The problem is that when I calculate the rate, if I understand correctly, it must be 10^-8 which is impossible to configure (I try to make 1TKN == $10-$15).

However, I have already seen codes that allow the use of fractions to reduce the 18 decimal places.
(Here is a link to a code with which I was able to tweak fractions and thus perform negative rates:
Do you think that with a code using Openzepplin it is possible to use negative rates?

here is my code (I use Remix.IDE):

pragma solidity ^0.5.0;

import "";

contract MyCrowdsale is Crowdsale {

    uint256 rate,        // rate, in bits
    address payable wallet,  // wallet to send Ether
    IERC20 token           // the token

    Crowdsale(rate, wallet, token)

Kind regards.