Access denied on external storage: application not found

I purchased an Android car head unit (K4811) and I cannot access a USB storage device that I plug in (32 GB, FAT32 or exFAT).

I think that this comes from the absence of a program allowing elevation of rights, because by downloading FileManager+, when I select the USB key, I obtain this message:


But if I click "OK", I get the error "Application not found":


However the application has storage access rights...

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