when I launch minecraft, it says:unable to save download. how do I fix this?

We are unable to diagnose your issue due to the lack of proper debug information. Please review the guidelines before reporting issues.
In case of a game crash, please also attach the crash log from [minecraft/crash-reports/crash-<DATE>-client.txt|http://hopper.minecraft.net/help/guides/finding-minecraft-data-folder/].

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