Up Sah 19.20 to 19.22 have been inter-alia discussed here. It requires fine tuning. Hence this post.                                                                                                                    Upadesha SahAsri 19.20 discusses the status of duality. It has neither birth nor absence of birth. Both have contradictions as explained below. Thus, duality is neither existent nor non-existent. It is neither real nor unreal. In VedAntic terminology, it is mithyA.
1 Suppose there is birth of duality, i.e., there is a cause of its birth. It may be an existent cause or a non-existent cause. A non-existent cause is a non-starter and cannot give rise to an effect. An existent cause can be either (a) non-duality or (b) duality.
(a) The cause cannot be non-duality, because the nature of non-duality is changelessness.
(b) If there is birth of duality A from duality B, then there is a question of birth of duality B and so on. It leads to infinite regress which is logically unacceptable.

2 Suppose duality has no birth. That is to say, duality always exists meaning thereby that it is changeless. Changeless duality is not experienced. It is non-existent. Therefore, the world of duality is mithyA though it is experienced.

There is another explanation (Up Sah 19.21). Production of any entity requires material and action.   According to satkArvAda, the created item (effect) exists in the material (cause), otherwise an entity can be created from anything. AsatkArvAda is coming into existence of a new thing which is non-existent in the cause. A non-existent thing ‘arrives’ is grammatically incorrect. ‘Arrive’ is a verb for which a subject is needed. A non-existent thing cannot be a subject. The argument that because of an intimate relationship between clay and the non-existing pot, the pot is created, is refuted for a relationship with a non-existing entity is inconceivable. (Up Sah 19.21)

Existence and non-existence are mutually exclusive categories (BG 2.16), otherwise, it is not possible to know which is the cause and which is the effect and cause-effect relationship is unascertainable like two sides of a weighing balance which alternately go up and down (Up Sah 19. 21). Moreover, there would be (undesirable) chaos where anything can come into existence from anything.

Existence is not a destination. A non-existent entity cannot travel, and an existent entity need not travel to existence. Since existence and non-existence are fixed categories, AtmA convinces the mind that nothing is born (Up Sah 19.22). Creation (duality) is mithyA.