I have collected almost 10,000 tweets for my research and just realised i forgot to get the Tweet ID of each tweet. For each tweet, I got their user_name. Is there a way I can retrieve my tweets IDs?

Did you literally record only the username, or any other information? How did you collect the tweets, i.e. via what endpoint?

I used the developer API to retrieve part of the the tweets, and I also manually collected some of the tweets (since the API only returns tweets from the past 7 days), i.e I used keywords in the Twitter search engine to get relevant tweets, copied and pasted them in my csv file. Yes, I only have Tweets and their usernames in my dataset now.

So, you’ve said you have “part” of the tweets" - what parts? If you’ve collected information that is (likely) unique, you might be able to search the user timelines to retrieve the IDs in some fashion.

The API doesn’t only return tweets from the last seven days - the search endpoints do, but you can retrieve older tweets to an extent using other endpoints.

Anyway, you need to give more specifics here. Specifically what information do you have in your CSV file, besides username?