The problem I encountered is related to the use of the _id field in MongoDB. The _id field is a unique identifier for a document, and its value can be of different types such as ObjectId, String, or even Mixed. In my case, the _id field is of type Mixed, which means that it can contain values of different types. When I tried to update a document using the _id field, I passed the value of _id as a string, but MongoDB was not able to find the document because the value of _id in the database was not a string. To fix this problem, I had to make sure that the value of _id that you’re passing to MongoDB matches the data type of the _id field in the database. In my case, I tried using the mongoose.Types.ObjectId() function to convert the string value of _id to a valid ObjectId, which can be used by MongoDB to find the document, but failed. This was causing the update to fail and return an error saying that the course was not found, even though it actually existed in the database.