How to change the error message for Forminator log in form?

Hi @master1234321,

I hope you are keeping well and thank you for reaching out to us.

The following workaround should help you with customizing the login error messages.

<?php

remove_filter( 'authenticate', 'wp_authenticate_username_password' );

add_filter( 'authenticate', 'custom_error_formi_loginScreen', 20, 3 );
/**
 * Remove WordPress filer and write our own with changed error text.
 */
function custom_error_formi_loginScreen( $user, $username, $password ) {
    if ( is_a($user, 'WP_User') )
        return $user;

    if ( empty( $username ) || empty( $password ) ) {
        if ( is_wp_error( $user ) )
            return $user;

        $error = new WP_Error();

        if ( empty( $username ) )
            $error->add( 'empty_username', __('<strong>ERROR</strong>: The username field is empty.' ) );

        if ( empty( $password ) )
            $error->add( 'empty_password', __( '<strong>ERROR</strong>: The password field is empty.' ) );

        return $error;
    }

    $user = get_user_by( 'login', $username );

    if ( !$user )
        return new WP_Error( 'invalid_username', sprintf( __( '<strong>ERROR</strong>: Invalid username.' ) ) );

    $user = apply_filters( 'wp_authenticate_user', $user, $password );
    if ( is_wp_error( $user ) )
        return $user;

    if ( ! wp_check_password( $password, $user->user_pass, $user->ID ) )
        return new WP_Error( 'incorrect_password', sprintf( __( '<strong>ERROR</strong>: The password you entered for the username <strong>%1$s</strong> is incorrect.' ),
        $username ) );

    return $user;
}

To change the error message mentioned in the screenshot provided, please replace the following from the above code.

<strong>ERROR</strong>: Invalid username.

The code could be added using a mu-plugin. I hope the following guide helps you further: https://wpmudev.com/docs/using-wordpress/installing-wordpress-plugins/#installing-mu-plugins

Please test the code on a staging or development environment before pushing it to the live website.

Kind Regards,
Nebu John

Thanks Nebu John, that worked!