On 20th of April I sent $154.79 worth of stellar lumens coins to external wallet without the memo. The transaction indicated completed and there was no credit to my external wallet account. Please help, how do I recover this coins
If you send a payment to an exchange that requires a memo and forget to include the memo when you do, the best thing to do is to contact the exchange's customer support. It is likely that they will be able to help you.
Generally, exchanges that require memos have a pooled Stellar account for multiple users, and they use the memo field to map the payment to a specific user's balance in their internal database. Often, they can work with you to find the transaction and credit it to your balance in their internal system.
For more information on how that works, check out this guide: https://medium.com/stellar-community/help-i-forgot-my-stellar-memo-d62b3cc9c2f7