Although the Standard Model of particle physics has been a very successful theory in explaining a wide range of measurements, there are still many questions left unanswered, like incorporation of gravity into SM, radiative corrections in Higgs masses, matter-antimatter asymmetry and the existence of dark matter candidates. One of the possible solutions to address these challenges is the extension of the present SM by incorporating an additional Higgs doublet. This search aims at exploring the presence of a neutral scalar ($H$) or pseudoscalar ($A$) heavy Higgs boson as predicted by the Two-Higgs-Doublet-Model (2HDM) produced in an association with a pair of top quarks with the heavy Higgs boson decaying further into a pair of top quarks. This analysis uses proton-proton ($pp$) collisions at a center-of-mass energy of 13 TeV, utilizing data collected by the ATLAS detector at the Large Hadron Collider (LHC). In the final state, events featuring exactly one or two oppositely charged leptons (electrons or muons) are taken into account. In order to improve the modeling of the most dominant background originating from top-antitop production in association with jets, data-driven corrections are applied. It includes the performance of a powerful Graph Neural Network classifier used for signal-background discrimination. This search is combined with the previous search performed by the ATLAS experiment in the multilepton channel and the upper limits are estimated on the production cross section for $H/A$ with mass ranging between 400 GeV and 1000 GeV in the alignment limit. Additionally, a model that predicts the generation of a color-octet scalar, known as sgluon decaying into a pair of top quarks, is constrained by the results.