Synchronicity is an interesting philosophical idea; unfortunately there is no evidence that it actually exists. Therefore, there are some people who consider Jung's theory to be pseudoscientific.
Robert Todd Carroll, in his book "The Skeptics Dictionary," notes that "even if there were a synchronicity between the mind and the world such that certain coincidences resonate with transcendental truth, there would still be the problem of figuring out those truths. What guide could one possibly use to determine the correctness of an interpretation?" There is no scientific or objective way to determine whether synchronicity is valid or not; it's all subjective personal opinion and experience and flexible definitions.
You can read more on this on this LiveScience webpage
I also commented on synchronicity within an article I wrote in May
Many of Jung’s theories are considered to be pseudoscience. One example is synchronicity, his idea that meaningful connections in the world manifest through coincidence with no apparent causal link. This is what Jung referred to as the acausal connecting principle (Nickell, 2002). This is because these ideas culminated from his ideas about the paranormal. Despite his own experiments failing to confirm the phenomenon (Shermer & Linse, 2002) he held on to the idea as an explanation for apparent ESP (Jung, 2013).
##References
Jung, C. G. (2013). Synchronicity: An Acausal Connecting Principle. London: Routledge.
Nickell, J. (2002). “Visitations”: After-Death Contacts. Skeptical Inquirer 12(3) Retrieved from https://skepticalinquirer.org/newsletter/visitations_after-death_contacts/?/sb/show/visitations_after-death_contacts
Shermer, M. & Linse, P. (2002). The skeptic encyclopedia of pseudoscience (Vol. 1). Santa Barbara, CA: ABC-CLIO.