ติดตั้ง Ubuntu เท่านั้น (ไม่มีบูตคู่) SSD+HDD

Yes, you did install it twice. Details to know: The location of the ESP isn't relevant. You can keep it in the HDD provided that drive has the first priority in UEFI BBS boot order but then you MUST use something else and define at least / (root), the only partition needed, on the other drive. Probably better to use GParted in a live session and delete everything. Then change the boot priority to the SSD and then proceed with a fresh installation.

Can you give me some instruction to do it with my current situation?

I already did. More info here: https://askubuntu.com/questions/6328/how-do-i-install-ubuntu

Is it not using ssd right?

Can i use only one single boot?

You should install it on the SSD and use the HDD only to save stuff. The SSD is much faster therefore better for the OS and programs whereas the HDD is cheaper to save big files (e.g. videos) for which the drive's speed is much less important or not at all. And having only one OS installed is what obviously makes sense. So, again, in a live session you can delete everything a start over making sure to select the SSD this time.

what do you mean by live session ? btw i posted new question https://askubuntu.com/questions/1359760/how-to-install-ubuntu-on-ssd-hdd-single-boot-only-ubuntu

You should post duplicates. "LIve Session" is what you have when you boot the installation media and select "Try Ubuntu"

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