You can create a new dimension removing the spaces at the beginning
Have you tried use Trim([your dimension field]) ?
Thank you all for the answers!
I tried all solutions, but I'm still a beginner with Tableau so the third one was the easiest for me.
In case anyone else encounters the same issue, I also found that by connecting my Excel sheet to my Tableau data source, I was able to edit it directly in Excel and have the changes made to Tableau without having to re-import the data, which would make me lose my work.