That means the kernel does not even see something was plugged. Here is what
gets appended to dmesg's output if I connect one of my SDXC cards (the
numbers between brackets are timestamps, seconds since the boot of the
kernel):
[ 2391.375282] mmc0: cannot verify signal voltage switch
[ 2391.496355] mmc0: new ultra high speed SDR50 SDXC card at address e624
[ 2391.500490] mmcblk0: mmc0:e624 SL64G 59.5 GiB (ro)
[ 2391.512435] mmcblk0: p1
Has the card reader ever worked? It may be unplugged or dysfunctional. You
can always try to update the kernel, but I very much doubt it is the problem
here: https://jxself.org/linux-libre/
