This beta sideload needs latest September Patch while Samsung don't push it to non beta regions on purpose.
First you need to have the September version. Second is to have the latest version of adb.
i also encounter this although i already have Sept build (G998BXXU3AUHB) and the latest adb. :'(First you need to have the September version. Second is to have the latest version of adb.
This is required version.
The latest version of the adb can be downloaded from the following link.
If you meet the requirements, I can't help you much more. If you have the same problem, the build date of Aug 17 is the date that corresponds to the auhb version, the necessary version is the auhd with the build date of Aug 19.