I am running low on ideas.
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Back to basics: double-check/reinstall the drivers, the setup of the power supply, and the correct board etc. etc. This issue is caused by something fundamental/basic.
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There is another post that is working on the LORA EXPLORABLE with other problems, but maybe that person is better help than me https://forum.sparkfun.com/viewtopic.php?f=99&t=59421
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Try to install the Sparkfun bootloader. you can get that from https://github.com/sparkfun/Apollo3_Uploader_SVL
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Maybe you have bad luck and 2 boards and should return them.