Microchip have dropped MPASM/MPASMX as of MPLABX 5.40. It's no longer included.
MPLABX is 64-bit only now, and MPASM is/was a 32-bit program.

They have a new 8-bit assembler, pic-as.exe and it's not compatible with MPASM. You'll have to stick with MPLABX 5.35.