Many Windows PCs ship with only one visible C drive. This is normal. If you want a D drive for file organization, shrink C drive and create a new partition from the free space.

Step 1. Select C drive on the system disk.
Step 2. Choose Resize/Move and shrink C drive to create unallocated space.

Step 3. Preview the resize and click Proceed.

Step 4. Select the unallocated space and choose Create.

Step 5. Assign D as the drive letter if available, choose NTFS, and click Proceed.

Having only C drive is not a problem. Split it only if a separate data partition makes your files easier to manage.