Register after Purchase

-- HikaShop version -- : 4.4.3
-- Joomla version -- : 4.0.3
-- PHP version -- : 7.4.15
-- Error-message(debug-mod must be tuned on) -- : Please note that you have the guest checkout activated for your checkout but products with a 'User group after purchase', like the one you just saved, can't be purchased by guest customers.

Hello
I activate Guest mode for Purchase, when i want to save product with setting "user group after purchase", I get this error:

Please note that you have the guest checkout activated for your checkout but products with a 'User group after purchase', like the one you just saved, can't be purchased by guest customers.

What do I do now?

H,i

Well, it's not an error. It's just a message to tell you that if someone tries to checkout as a guest, without registering/logging in, then they won't be able to purchase products with the "user group after purchase" setting set to something as HikaShop cannot add a user group to a user account that doesn't exist in the first place.
So you can't have "register after purchase". The registration needs to happen before/during the checkout.
So you would have to configure your HikaShop checkout, under the "Checkout" tab of the HikaShop configuration to allow the customer to register/login.