As we’ve discussed in other articles, machines with Windows 10 installed frequently have multiple partitions. One or more of those partitions is typically labelled as a “recovery partition”.
While in the past you may have seen such partitions assigned a drive letter like D:, there’s no requirement that it always be that letter. In fact, there’s no requirement that it be assigned a drive letter at all.
Recovery partitions not having a drive letter is actually a good thing.