You have inquired why Caitanya Mahaprabhu has not mentioned anything about accepting a Spiritual Master in His Siksastaka. But perhaps you have missed the point that He says amanina manadena kirtaniya sada hari. This means one has to chant the Holy Names of Krishna, becoming humbler than the straw, and more tolerant than the tree. So who can become humbler than the straw unless he accepts a Spiritual Master?
Letter to Jaya Govinda, 4 July, 1969