Most likely you are using an install file which is not for installing via ADB sideload.
The install files from the download pages currently match the instructions given here … https://doc.e.foundation/devices/FP3/install .
You can get download links for install files suitable for ADB sideload from https://ota.ecloud.global/api/v1/FP3/stable or https://ota.ecloud.global/api/v1/FP3/dev (see the “url:” lines respectively).