I am using MS-DOS 6.22 on a VirtualBox VM. I needed to quickly build a small program and, in order not to constantly throw it on a floppy disk image to run under MS-DOS, I decided to install and compile it immediately on a virtual machine.
At first, I tried to use the version downloaded from the developer's site, but I got a message: "Program too big to fit in memory!". The same message is returned even if you recompile FASM from the source code.
It is worth adding control overflow the length of segments in MZ applications.

It definitely works on my MS-DOS 6.22 (both on actual machine and on VM), so you may have some additional issues. How much conventional memory do you have available? (You can check that with MEM command.) Also, have you tried FASMLITE.EXE?

PS. You are not, by any chance, trying to run a Linux executable there, are you? That's another possible circumstance that comes to mind, where you'd get this kind of error message (because when .EXE file is in format unknown to DOS, it tries to load it as a .COM file, which is limited to 64K).

I checked it on a real machine and everything really works. But it doesn't want to run on VirtualBox. I allocate 64 MB of memory for the virtual machine, but in MEM.EXE it turns out 640 kb and 52 kB is occupied. It seems that everything should be in order. On a real machine, MEM shows even less - 98 kb is occupied.

No. I checked it several times. There is a whole MZ variant of FASM on the VM. Strange, but refuses to work only on it.

P.S. Perhaps the problem is in the version of MS-DOS that is used on VirtualBox. There are a lot of edited versions of the operating system on the Internet now. It is not surprising that it did not work on the first MS-DOS 6.22 image that came along. When there is more time to check, I will try the MS-DOS that remained on the floppy disk from Windows XP. I can't find a working FDD yet.

try here for ms-dos
https://winworldpc.com/product/ms-dos/1x

ver 1 to ver 7.1