You tried this?
There are 2 files:
"RTP" and "Descendants vX.X"
# 1 You have the RPG Maker VX Ace RTP installed on your PC #
1.1-You don't need to download the "RTP" and just download most recent version of "Descendants vX.X".
1.2-Once downloaded just extract/execute the downloaded file.
1.2-After that execute the Game.exe in the folder, and play.
# 2 You don't have the RPG Maker VX Ace RTP installed on your PC #
2.1-You will need to download the "RTP" and the most recent version of "Descendants vX.X" file.
2.2-Once downloaded, extract/execute the two files on different folders.
2.3-Once extracted, COPY the"Descendants vX.X" folder files into the RTP folder and REPLACE all the files in it.
2.4-After that execute the Game.exe in the folder, and play.
*You can do this for bug fixes too, for example you downloaded the v0.2.00 and the v0.2.02, you just need to replace the old files with the next version files, but this don't work with v0.2.00 to v0.3.00.
You can download the RPG Maker VX ACE RTP here.