https://github.com/bitcoin/bitcoin/blob/57a8fa9c3b4452ef7a9d9ed4b957b3888f542fb0/src/script/script_error.cpp#L60-L61  
It should be `Non-push operators are not allowed in signatures` or `Only push operators allowed in signatures`.
#include <script/script_error.h>
const char* ScriptErrorString(const ScriptError serror)
{
    switch (serror)
    {
        case SCRIPT_ERR_OK: