Hello @Are Kaveri 
Try with the below Onchange Client script on Impact field:

function onChange(control, oldValue, newValue, isLoading, isTemplate) {
 
    if (isLoading) {
        return;
    }

  
    var ci = g_form.getValue('u_configuration_item').toString();

    // Check if the configuration item is 'ABC' and if u_impact is set to 'Yes'
    if (ci === 'ABC' && newValue === 'Yes') {
        // Show the error message on the u_impact field
        g_form.showFieldMsg('u_impact', "The impact should not be set to Yes when Configuration item is ABC", 'error');
    } else {
        // If the condition is not met, hide any previous error messages on u_impact
        g_form.hideFieldMsg('u_impact');
    }
}

Please Mark Correct  ✔️ if this solves your query and also mark Helpful  👍 if you find my response worthy based on the impact.

Regards,
Shruti