Make sure her disk doesn’t have any unallocated partition. Then refer to this link for solution
Enabling BitLocker fails with “BitLocker Setup could not find a target system drive. You may need to manually prepare your drive for BitLocker”
http://adriank.org/enabling-bitlocker-fails-bitlocker-setup-find-target-system-drive-may-need-manually-prepare-drive-bitlocker/
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