could you check if any "Installer.log" file is produced under:
in addition, you could try to download our MSI installer package (from the same place as the EXE), and start it from command prompt with the following:
msiexec /i "C:\JChem_For_Office_16.5.3000.824.x86.msi" /L*V "C:\log\example.log"
Note: You should substitute the location of the msi file, and where you want to place the log.
When it is done, please post the logs here, or you could aslo send us by mail to:
jc4o-support _at_ chemaxon.com