Please first make sure that your SD is not in "read-only" mode.

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Then, please go to the app Settings - Change storage location and check if the SD option could show up.

If it does, please try changing it to Internal Storage and then switch back to SD card.

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If not, it is possible that your SD card are not able to read successfully. You could try remounting (pull out and plug in), reformat your SD card. If the problem persists, we're afraid that you may need to replace your SD card.