How to format a drive encrypted with Bitlocker?

Start a Linux Live CD like the one from GParted and delete all (Bitlocker enabled) partitions.

If that doesn't help there is still the possibility to wipe the complete disk - after overwriting the first few megabytes the HDD will be recognized as fresh new HDD by Windows. You can do that for example using DBAN but don't forget to disconnect all the other HDDs before using it - otherwise you may delete the wrong HDD and loose all your data.

Press SHIFT-F10 or hit 'repair' in from the Windows installation to open up the command line, then execute the diskpart command and delete the partition, e.g.: list disk, select disk 0 or any other identifying the correct disk, list partition, select partition 1, or the encrypted one , in case there are multiple partitions, then delete partition override.

You can then resume the install procedure normally to repartition and format the drive.