Does self-actualisation happen during the journey or after achieve the goal Let say one is talented in music, and wants to become a great musician. Does self-actualisation refer to mental state during the journey of pursuing goal, or the mental state after achieve the goal. Originally I thought it's the latter. Recently I read about the concept of arrival fallacy, which casts doubt over my understanding.
Self-actualisation in Maslow Theory (wiki):

the desire for self-fulfillment, namely the tendency for him [the
individual] to become actualized in what he is potentially. This
tendency might be phrased as the desire to become more and more what
one is, to become everything that one is capable of becoming.

Arrival fallacy (article link):

“Arrival fallacy is this illusion that once we make it, once we attain
our goal or reach our destination, we will reach lasting happiness,”
said Tal Ben-Shahar, the Harvard-trained positive psychology expert
who is credited with coining the term.
Short answer
According to Maslow, you reach self-actualisation when you reach the goal.  Not juring your journey.  The arrival fallacy refers to the fact that if you reach that goal, it doesn't necessarily mean that you will stay there.
Take away any of the levels below self-actualisation, and you will not reach the top goal. If once you reach there, one of the lower levels goes for some reason, you will lose the position of having been self-actualised.
Maslow himself said:

Though, in principle, self-actualization is easy, in practice it rarely happens (by my criteria, certainly in less than 1% of the adult population.  For this, there are many, many reasons at various levels of discourse, including all of the determinants of psychopathology that we not know. (Maslow, 2013)

Full answer
My answer to Is self-actualization really a need? gives a clue.
We need to determine what self-actualisation actually is.  Maslow described it (emphasis mine) as follows: (Maslow, 1943)

This term, first coined by Kurt Goldstein (1939), is being used in this paper in a much more specific and limited fashion. It refers to the desire for self-fulfillment, namely, to the tendency for him to become actualized in what he is potentially. This tendency might be phrased as the desire to become more and more what one is, to become everything that one is capable of becoming.
The specific form that these needs will take will of course vary greatly from person to person. In one individual it may take the form of the desire to be an ideal mother, in another it may be expressed athletically, and in still another it may be expressed in painting pictures or in inventions. It is not necessarily a creative urge although in people who have any capacities for creation it will take this form.
The clear emergence of these needs rests upon prior satisfaction of the physiological, safety, love and esteem needs. We shall call people who are satisfied in these needs, basically satisfied people, and it is from these that we may expect the fullest (and healthiest) creativeness (Kardiner, 1941). Since, in our society, basically satisfied people are the exception, we do not know much about self-actualization, either experimentally or clinically. It remains a challenging problem for research.

Maslow's hierarchy is visually put together as follows:

Image Source: Wikimedia Commons Under Creative Commons CC BY-SA 3.0)
In the triangle's levels, Physiological, Safety, Love and Belonging, Esteem, and Self Actualization which are on the left are the goals at each level. The items inside each level are your needs for each goal. If, for example, you don’t have all your needs for the physiological goal level, you cannot have safety, even if you have all the prerequisite needs in the safety level.
To attain the sixth level or stage: self-actualisation, is the goal of Maslow's Theory, not a need.  Take away any of the levels below self-actualisation, and you will not reach the top goal. And as stated by Maslow (1943) — see above — and in the TLDR summary of @NickStauner's answer to Does evidence support Maslow&#39;s Hierarchy of Needs?,

One sequence can't fit all (it didn't claim to), but it makes sense in general, and so do the exceptions, and so do other motive models.

Maslow himself said:

Though, in principle, self-actualization is easy, in practice it rarely happens (by my criteria, certainly in less than 1% of the adult population.  For this, there are many, many reasons at various levels of discourse, including all of the determinants of psychopathology that we not know. (Maslow, 2013)

also, the fact that &quot;most of us function most of the time on a level lower than that of self-actualization&quot; he called it the psychopathology of normality. (Loevinger, 1976)
References
Goldstein, K. (1939). The organism: A holistic approach to biology derived from pathological data in man. Salt Lake City, UT: American Book Publishing.DOI: 10.1037/10021-000
Loevinger, J. (1976) Ego Development: Conceptions and Theories (Jossey-Bass Behavioral Science Series). San Francisco, CA: Jossey-Bass Inc
Maslow, A. H. (1943). A theory of human motivation. Psychological Review, 50 (4), 370–96.DOI: [10.1037/h0054346][14]FREE via http://psychclassics.yorku.ca/Maslow/motivation.htm.
Maslow, A. H. (2013). Toward a psychology of being. Simon and Schuster [eBook].