You have a uSD card with a 32-bit operating system, which is located in the uSD card slot of the built-in RPi4.
If you downloaded the sb64p3.img.xz compressed file to the Downloads folder, then unfortunately you do not have enough space to unpack it.
If so, you will need to transfer it to your PC via an intermediary USB stick.
You will need a new 32GB or 64GB uSD card, onto which you will first need to unpack the downloaded sb64p3.img.xz file (size 2,923,375,804B) (size 10,314,428,928 B) so that you can install it on the new uSD card with the Balena Etcher program. To do this, you will need a uSD card reader, and you will need to insert the new card into it.
You do all this under Windows OS, but if you are on Linux you can do it there too.
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I didn't think I would have to describe what you have to do step by step.
This device is an SBC (single board computer) controlled SDR radio, so the operation depends heavily on the correct settings. You need to know how to do things under Debian. You need to use the File Manager.
So you install Balena with Etcher on the uSD, inserted into a card reader. This takes 20-30 minutes depending on the uSD card.
When the program has finished checking, you can remove the card from the card reader and now replace the one in the device with this new 64-bit one.
Mark what you removed as the original 32-bit one. Close the device back and power it on. Wait for the system to recover.
Check that if you start sbitx, v4.2 will run. Exit and start sBITX-Toolbox and first perform an upgrade, wait patiently until it finishes.
Start Toolbox -> Update Manager. Here select sbitx app update and wait until it finishes.
What was left out: a Hotfix was made for the Image file, this is a bug fix for qsstv and usb headset.
Run this in Terminal with the command described on the ELF page.
I know this is a lot of information, but I can't make it shorter according to the step-by-step.
Read back my previous messages, what was left out is described there.
HA3HZ