It appears that the reason your donation receipt is not tax deductible is due to the fact that the charity does not have deductible gift recipient (DGR) status as issued by the Australian Taxation Office.
However, whether the receipt is tax deductible is entirely dependant on the school/ charity itself. The charity needs to have tax deductibility, which only certain organisations are entitled to, in order for any receipt issued on their behalf to be tax deductible. If your receipt is not tax deductible, it will explicitly say it on the donation receipt for your reference.
My best suggestion here would be to contact your charity to enquire further about their DGR status.
Thank you for using everydayhero to support the cause you care about and if you have any other questions please feel free to let us know.