Firstly, the build of FORScan you are using is very much out of date. The newest build can be downloaded here:
https://forscan.org/download/FORScanSetup2.4.3.test20210421.exe
This build contains a “recovery mode” that might be able to help. You are updating a “G” series APIM and these are known to be quite finicky when it comes to firmware updates. I’m not all too familiar with the process so I can’t be of much more help unfortunately.