I assume it was a pass bought in the sale during June this year, these passes were sold under the condition they were only valid for starting after the peak summer period, so 1st activation date 01 Sep 2024.

Did you buy your pass during the summer sale? If yes, there was an information that these passes are only valid for dates form September 1st onwards.

Edit: Al_G was faster

You can make an exchange of passes. Buy a new full price Interrail pass and then follow the instructions for making an exchange, see link below. Otherwise you must wait with your travel until the 1st of September. 

https://www.interrail.eu/en/interrail-passes/ordering-info/refund-exchange-policy