TWRP tells me: adb: sideload connection failed: insufficient permissions for device: user in plugdev group; are your udev rules wrong? ❯ adb sideload lineage-17.1-20201108-nightly-guacamole-signed.zip
* daemon not running; starting now at tcp:5037
* daemon started successfully
adb: sideload connection failed: insufficient permissions for device: user in plugdev group; are your udev rules wrong?
See [http://developer.android.com/tools/device.html] for more information
adb: trying pre-KitKat sideload method...
adb: pre-KitKat sideload connection failed: insufficient permissions for device: user in plugdev group; are your udev rules wrong?
See [http://developer.android.com/tools/device.html] for more information
This is because TWRP doesn't support sideloading on the One Plus 7 Pro. From the official port page

If you don't download the zip file to your device first for any reason, you will need to adb push the zip to the device because MTP is not currently working on the OnePlus 7 Pro in TWRP. If you are unable to use adb, you can also use a USB stick with an OTG cable, if you have those handy.