The evaluation "license" is valid for 30 days. In practice this means that you are not able to create a bootable USB stick and/or erase a computer after the 30 day period. If this happens, please contact evalsupport@blancco.com.
If you have just created the bootable USB stick within the 30-day evaluation period, but you are getting an error message on the Blancco Drive Eraser side, related to software expiration, make sure that the computer's internal clock is set to the correct time from the BIOS.