Hello,
What level are you? There is a level requirement no both Missions General Mobilization and the packs that support this event. Sadly, they are not the same level, so if you've just gained access to the event you may not be able to purchase the packs.

If you are a high level, then this wouldn't be the issue, and instead you will need to create a ticket with the customer support system and ask for help with your account. Here is the link.