At the beginning, make sure that the contact base is in the correct format - your file should be in a .xls, .xlsx, or .csv format. For the contact base to upload correctly, you must also fill in all the mandatory fields - the name of the medium and email address (filling in the rest of the fields will help you identify contacts during distributions).
You must correct all the cells that are highlighted in red. The red highlight indicates that the cell contains invalid data and that the contact base will not load properly. The most common errors are using Polish characters in email addresses and unnecessary spaces.