Does this occur when you try to run the RS3 client normally? IE: without RuneMate open, or launching RS3 client from within RuneMate?
Try the normal things to help fix it:
Uninstall RuneScape
Uninstall RuneMate
Uninstall all Java versions
Delete all cache files for all of the above
Download fresh copy of RuneMate
Let it install appropriate Java version automatically
Download & install RuneScape (legacy client)