Why is growth depressed when a diet has adequate protein but lacks folic acid (or vitamin b12)?

Folic acid is vitamin B9, not B12. It is very important for the body because it is part of cell division and growth. Folic acid participates in carrying one-carbon groups for methylation reactions and is also part of the synthesis of nucleic acids (DNA, RNA). So, although there might be a sufficient intake of proteins, without folic acid there is no DNA to integrate the cells and continue division. Folic acid is an essential vitamin because it is only part of the organism when it is absorbed from the diet.