[Konstantinos]

Before:

pawn Код:

error 008: must be a constant expression; assumed zero

shows the number line inside parentheses; for example: (line_number_here).

Goto that line and paste it here.

By the way, wouldn't be easier to just do:

pawn Код:

if(isnull(cmdtext)) return SendClientMessage(playerid,COLOR_RED,"Usage: /pm [id] [message]");
// Assuming cmdtext is similar to "params".