There are 2 reasons why you can face this error:
The best way to solve the issue is to wait for an hour and try to activate your license key again. If it doesn’t help, try the manual activation process described below.
Step 1
Open up functions.php file of your active theme and paste the following code:
// Use this code to activate Extra Shortcodes add-on
update_option( 'su_option_extra-shortcodes_license', 'ENTER-YOUR-LICENSE-KEY-HERE' );
// Use this code to activate Additional Skins add-on
update_option( 'su_option_additional-skins_license', 'ENTER-YOUR-LICENSE-KEY-HERE' );
// Use this code to activate Shortcode Creator (Maker) add-on
update_option( 'su_option_shortcode-creator_license', 'ENTER-YOUR-LICENSE-KEY-HERE' );
Step 2
Replace ENTER-YOUR-LICENSE-KEY-HERE with your license key;
Step 3
Open any page of your site (no matter front page or any admin page). This will run the code in functions.php file;
Step 4
Now your license key successfully added to database and you can remove added code from functions.php file;
Step 5
Check your license key status by navigating to Dashboard – Shortcodes – Settings page.