@Cernel nah, I think we should be fine

While I can't provide a solution, I might be able to present an idea that might help.

I dug through the code and found that there's only a single error message for all the image variations of a unit. Therefore if mech_infantry_hit doesn't exist, you get the same error message as if mech_infantry doesn't exist.

I really have no idea on how to make maps, but I hope this helps

I think the problem is that @Ethan-Yang 's java version is probably 11, and that the current 1.9 version doesn't support this java version.

So you can either downgrade your java, or use the lates pre-release, but this version is incompatible with the current stable, so online play won't be possible

In order to fix the MDS (ZombieLoad) CPU vulnerability, our server hoster will restart a couple of our servers in the coming two weeks.
The planned schedule is as follows:

• 2019-06-25 3:00:00 AM UTC - TripleA-Forum
• 2019-06-27 2:00:00 AM UTC - bot35_frankfurt_de
• 2019-06-29 1:00:00 AM UTC - bot55_london_uk
• 2019-07-01 5:00:00 PM UTC - bot65_tokyo_jp
• 2019-06-26 4:00:00 AM UTC - PreRelease

They are scheduling a downtime window of 2 hours but assume the actual downtime will be much less.
The servers should start the bots again on their own automatically, but we had some issues in the past with this process (which hopefully should be fixed by now, but you never know), so we might be short on a couple of bots for some time in the worst case (for me this time window is somewhat in the middle of the night, so I can't stay up that late on a weekday, hopefully one of the other devs can have a look at this).

Sorry for the inconvinience

No problem.
Yes, 1.10 is incompatible with 1.9 has to do with the way savegames are saved and such

Even without the installer you should be able to run triplea by using the provided zip downloads for the latest master version or the same asset file for the current stable.
If you unzip the folder, and have java installed on your system (latest master requires java 11, latest stable java 8 ) you should be able to run it either via command line or by double klicking.
But if you really insist on building triplea from source you can of course do that as well by checking out repository via git and running ./gradlew :game-headed:run, which should build it from source and execute it right away when you have java 11/8 installed on your system.

I could try running triplea on a 32 bit windows

Well I know that my father's PC is running a 32 bit installation of windows, even though the cpu supports 64 bit.
My assumption is that there are a lot of tech-illitrrate people with - in comparison - low-end PCs aka 4 GB RAM or less where 64 bit doesn't make a difference at all. So wether or not they have 64 bit windows is a 50:50 chance.