Each voucher and each event has its own conditions and sometimes these are not compatible. That is why, when you are about to apply a voucher, you will see them split in "Available" and "Not Available".
Even though you will only see the limitations of an event in the final breakdown when applying the voucher, you can see the conditions of the voucher before even looking at what plan you want.
To do this, just go to the section "vouchers" and click on each one. There you will be able to see for what, for when and how you can use them.
For more information about using vouchers in your purchases, click here.