The species problem is the set of questions that arises when biologists attempt to define what a species is. Such a definition is called a species concept; there are at least 26 recognized species concepts. A species concept that works well for sexually reproducing organisms such as birds may be useless for species that reproduce asexually, such as bacteria."

Source : Wikipedia

The problem of accurately defining what a 'species' is goes back at least as far as the 1870s.

Species. — No term is more difficult to define than 'species', and on no point are zoologists more divided than as to what should be understood by this word. Naturalists, in fact, are not yet agreed as to whether the term species expresses a real and permanent distinction, or whether it is to be regarded merely as a convenient, but not immutable, abstraction, the employment of which is necessitated by the requirements of classification

Source : H. A. Nicholson, A manual of zoology. 1872.

Despite decades of research, biologists do not agree on what constitutes a species. Several dozen definitions have been proposed."

Source : New Scientist April 2022

Even given the extreme accuracy of genetic profiling now available to taxonomists, many are resigned to the fact that an accurate definition of what a species actually is may never be practical.

Also see :Number of speciesplugin-autotooltip__plain plugin-autotooltip_bigNumber of species

"We demonstrate that after more than six decades, estimates of global species richness have failed to converge, remain highly uncertain, and in many cases, are logically inconsistent."

Source : Global species richness estimates have not converged for Trends in Ecology & Evolution.