I created the external User in Azure, when i give access to this user in the Site Collection "A" it say "Access denied" but when i try with another site collection it work! so the problem i think is only with the site "A". how i can configure it please?
Please have a check on the site collection feature with an administrator account. go to Site Settings->Site Collection features, find "Limited-access user permission lockdown mode" feature, make sure it is deactivated. If it does not help, try to remove the external user from the Site Collection user information list and re-invite the user to the correct group. Make sure the invitation process works properly.