I can't import leads to Zoho CRM!

I can't import leads to Zoho CRM!

I try to import only 6000 leads through the XLS file (CSV and XLSX). Zoho always responds: its impossible import more than 25,000 leads.

I tested an XLS with 20 records, and Zoho also replied: its impossible import more than 25,000 leads.

I am working on a free Zoho account.

What's going on?