Put you sql into a string an debug.print it - the error will be obvious;
Code:
Dim strSQL as String
strSQL = "UPDATE Matches SET Matches.Toss = Left([Matches.Toss], InStr([Matches.Toss], ", ") - 1) WHERE Matches.Toss Like "*, *""
Debug.Print strSQL
CurrentDb.Execute strSQL, dbFailOnError
Press ctrl+g to show the immediate window in the VBA editor it will display your string.
I suspect it should be something like
strSQL = "UPDATE Matches SET Matches.Toss = Left([Matches.Toss], InStr([Matches.Toss], ', ') - 1) WHERE Matches.Toss Like '*, *'"