Hi David, Thank you for posting your question here in Microsoft Q&A Forum! Let me help you out.
BitLocker isn’t letting you type letters because the recovery screen only accepts the 48 digit numeric recovery key. It’s always numbers only, usually in 8 groups of 6 digits, and you can ignore the hyphens. If it rejects letters, it’s because it’s waiting for that numeric key, not a PIN or password.
If you still don’t have the key, check every place it could be stored your Microsoft account, your work or school account if the device was managed, any printed copy, a USB backup, or your IT team if it’s a company device.
If you set it up with school or work account, it is worth checking here as well https://aka.ms/aadrecoverykey sign in with your work/school email credential in that case. Once you find the 48 digits just type the numbers in.
Best regards,
Angelique P.