Hello @gimena_sierra and welcome to the Canvas Community! If I understand correctly, students are able to upload some types of files, like PowerPoint, but not Excel. Is this correct? You might check and see if under your assignment settings .xlsl file types are an allowed format. In the example below from one of my courses, a student would not be able to submit an Excel file.
I have found this to be very effective in restricting uploads, so in the assignment above I included both .doc and .docx so neither would be restricted.
If .xlsx files are allowed and students still cannot upload the file, please let us know but it may also be a good idea to submit a help ticket through Canvas.
All the best!