Octal450 :
I performed short test flight from Paro to Lhasa, now it is working perfectly.
Again thank You.
miguel :
Will check and report.
EDIT - miguel :
Are You familiar with Simbrief workflow ? If not, here is tutorial (not mine) :
Did You followed instructions and enter Your username into Simbrief API window ?
You can use INIT REQUEST only without running engines. If You start engines, Simbrief is not allowed. And one more reason for Not found error - Your flightplan has depart, destination or divert destination that not exists in FGFS. For example, Navigraph has not for Thoise ICAO code VI57, but VITE, so flightplan VNKT -> VITE in Simbrief probably will not work in FGFS. I must test this case.
EDIT 2 :
Hmm, when Simbrief flightplan contains airport that doesn't exists in FGFS, error message is other - Simbrief download failed. I do not know where is Your problem. I hope, lego, Octal450 or merspieler will explain.