The structure of the IUPAC name is
name of cationic part + Name of anionic part.
Metal present in the anionic complex, so 'ate', must be added at the end of the metal name.
Oxidation state of Mn
Mn+4(−2)=−2=+6
The structure of anionic complex name is
ligand names + metal name(oxidation state)
K2MnO4
2+x–8=0
⇒x=+6
Oxidation state of Mn=+6
IUPAC Name is:
Potassium tetraoxidomanganate (VI)