Because TASM (even the last version) is a very DOS old program with no support for Windows LFN/VFAT/(call it what you want). It does only accept plain old DOS 8.3 file names.
One option to do something with that is to give TASM ADDTWO~1.ASM (assuming you have none other files with name beginning with AddTwo) as file name. Another - abandon long file names. The third - use something newer, perhaps MASM or FASM.