The fossil record is considered an incomplete record because it does not record the expected transitions between the different fossil groups.
Darwin's theory of descent with modification predicted there would be transitions between the different fossil groups. The general acceptance of Darwinian evolution created an expectation of finding numerous transition fossils showing slow uniform changes between the various fossil groups.
The universal failure to find the expected transitional fossils created the impression that the fossil record is incomplete. There are several methods to explain the lack of transitional fossils.
The transitional fossils existed but have been destroyed by erosion, earthquakes, and subduction zones.
The transitional fossils existed in only isolated geographical locations where the changes occurred. This explains why the transitional fossils are not found. This theory is called punctuated equilibrium
The transitional fossils never existed. The slow uniform changes predicted by Darwinian evolution did not occur. This is sometimes called the "hopeful monster " postulating sudden rapid changes in the fossil groups.
Darwinian evolution is wrong and the postulated changes of descent with modification did not occur.