Question

Red blood cells have no nucleus, then why can we call them cells?

Answer 1

Cells are the essential building blocks of all living things. The physical body consists of trillions of cells. Cells have many parts, each with a special function. These parts, called organelles, are specialized structures that perform certain tasks within the cell.

Complete answer:
Red Blood Cells are produced within the bone marrow and they have a nucleus once they are initially produced. Later, they lose their nucleus so as to accommodate more hemoglobin in order that they will transport more oxygen. Therefore they're still considered as cells.
(a) Red blood cells don't have a nucleus because it provides an outsized area in order that it can carry oxygen to all or any parts of our body. Calling something a cell doesn't mean that it should contain a nucleus. If the nucleus was present during a RBC then it might not be so supportive in carrying oxygen because it does.
When red blood cells (erythrocytes) are produced within the bone marrow, they initially do contain a nucleus. the shortage of a nucleus allows it to hold more oxygen. The cell is additionally more flexible, improving its ability to travel through capillaries that are about half the dimensions of the cell itself.

Note:
The functional unit in RBCs is Hemoglobin. It binds to Oxygen and $CO_2$ and carries them from one part of the body to a different. One Hb molecule can carry 4 oxygen/Carbon dioxide molecules. This function doesn't require a nucleus and thus, the RBCs work without it efficiently.