You are only allowed the change the custom URL up to three times a year and you should do the following steps in order to change this.
- Sign in to YouTube and go to your 'About me' page.
- Select the account with your YouTube channel in the top-right corner. You may have to click All of your Brand Accounts if your account is not in the list.
- Find your current custom URL under 'Sites' and click edit
.
- Next to the URL that you want to remove, click X.
- Click OK.
If you're eligible, you can claim a new custom URL immediately after removal, but it may take a couple of days before the previous custom URL stops directing to your channel.
With issues such as these its always best to follow the prompt that your seeing for example in your screenshot you should have clicked 'learn more' to find out more about the issue, as this would outline possible steps to you