Hi,
the '%' char is treated as a special format character. Please remove the '%' from your command line - then it should work.
No, %COMPUTERNAME% is standart variable for Windows. If я write FARM
TT-Miner.exe -A ETHASH -P 0x71a512963677f01cf44e564f43117fc02dfcd5d0.FARM@eu1.ethermine.org:4444
it is don't work too