why I can not download a macOS high Sierra installer?

Click on this link to read how to get different versions of macOS - https://support.apple.com/HT211683

You really aren't providing me with the information I need to help you effectively. is this computer completely without a bootable system? If it is, you will have to use Internet Recovery to get one onto it and then work from there to upgrade to what you want in the end.

If it is booting to something then it would be helpful to know to what. If you put in a bare SSD there is no recovery partition which is what you would normally you to jump-start installing a new OS.

If there is a bare drive in there then it will need to be formatted, but I feel I am just typing in a lot of information and trying to cover every single possibility because I don't really know what is needed here. That's enough for now.

What have you tried?

If you are completely without a boot system try:

How to reinstall macOS from macOS Recovery https://support.apple.com/HT204904

Select Internet Recovery. You may then have to upgrade but that's a separate step which I won't go into until I know more about what is actually needed here.

I still don't know what exact model and year Mac we are dealing with here! There's about two decades of iMacs out there. :-/

I haven't had to work with a bare boot drive in many years. I always format them in an external enclosure before putting them into my computer with the OS installed. I have also never had to use Recovery, and certainly not Internet Recovery. It is possible Internet Recovery may let you format a drive, but since I don't know what year computer you have it may or may not be able to do Internet Recovery.

To me it would be easier to partition it to GUID and format it outside the computer since it is bare. Now comes the question of which operating system version your other computer has on it. There's two formatting options depending upon which it has but I don't want to have to continue to write all the options for multiple scenarios and would prefer to know more about the hardware involved first.

She can not download from the apps store only Mojave and it not working with it.

It is a bear ssd with no boot, no recovery, not formated.

do i need to take it out and format it?