shouldn't the line which causes the error begin:
objPPTX
rather than

PPTX
?


Part 2:
objPPTX.ActivePresentation.saveas Filename:="C:\Users\s.kalantari\Desktop" & cell01 & ". PPTX"
(not tested. cell01 shouldn't contain any illegal filename characters)


I suspect there's a way to change file names without opening the file at all.