If last trade was BUY, next trade should be SELL

So what's stopping you?

Help you with what? You haven't stated a problem, you stated a want. Show us your attempt (using the CODE button) and state the nature of your problem.
          No free help (2017)

Or pay someone. Top of every page is the link Freelance.
          Hiring to write script - General - MQL5 programming forum (2018)

We're not going to code it for you (although it could happen if you are lucky or the problem is interesting).
          No free help (2017)

You can start learning to code using this link:

https://www.mql5.com/en/forum/296230

if(prev_position_type==POSITION_TYPE_BUY)
   {m_trade.Sell(Lot,m_symbol.Name(),Bid,0,0,NULL);  
    return;               
        }

if(prev_position_type==POSITION_TYPE_SELL)
   {m_trade.Buy(Lot,m_symbol.Name(),Ask,0,0,NULL);  
    return;               
        }