The Nikon 1.4x TC will only cause one-stop light loss, not 1.4 stops. So since your current lens is a constant f/5.6, it will make it a constant f/8. What body do you use? Not all bodies are capable of focusing at f/8. To understand, please read this Nikon article.
Which DSLR cameras are compatible with focusing at f/8? Df, D4, D4S, D5, D6, D500, D600, D610, D750, D780, D800, D800E, D810, D850, D7100, D7200, D7500.