Few checks for you:
1) Please ensure that your system's date format is in format dd/mm/yyyy and you have entered the dates in excel template in similar format.
2) You are trying to upload ".mtr" file only and not the excel file on the website. To get the excel file, right click when the template is open and click export.
In case you are still facing problem in uploading Capital Gain Records then you are requested to email your excel file to email@example.com with the Subject line 'Capital Gain Records'.
Our Experts will review and upload the data and intimate you to complete your Income-tax return. Please remember to quote your username and PAN in the email.