Thanks for reaching out. My name is, Zadee. I'm an Independent Advisor and a Microsoft user like you. I'll be glad to help you today.
Bitlocker has to be disabled first before you can extend the bitlocked drive, Georgi.
Similar questions on these threads or articles. https://social.technet.microsoft.com/Forums/lync/en-US/16b93e58-f808-4344-accf-e93499aa24d7/extending-partition-of-a-bitlockered-drive?forum=winservergen
https://www.diskpart.com/articles/resize-bitloc....
https://www.elevenforum.com/t/resize-c-d-partit...
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Best Regards,