As noted, MPASM is only available in MPLAB X IDE versions up to v5.35. Newer versions require migrating to the PIC-AS assembler.
On Windows, right-click the .exe and select "Run as Administrator." download mplab x ide v5.35
Download MPLAB XC8 v2.10 or earlier if you require legacy C90 standard support. As noted, MPASM is only available in MPLAB
