Hydrochloric acid can not be used to make the medium acidic in the oxidation reactions of  ${\text{KMnO}}_{\text{4}}$The reason is due to

Both HCl and  ${\text{KMnO}}_{\text{4}}$ act as oxidising agents 

${\text{KMnO}}_{\text{4}}$ oxidises HCl into ${\text{Cl}}_{\text{2}}$ which is also an oxidising agent

${\text{KMnO}}_{\text{4}}$ is a weaker oxidising agent than HCl

${\text{KMnO}}_4$ act as reducing agent in presence of HCl