This could be because you have not registered for Third Party Transfers at your bank or your Third Party Transfer limit is too low. Some banks like HDFC allow you to set up Third Party Payment limits up to Rs 10 lakh instantly. See HDFC Bank - Flexible Third Party Transfer limits on NetBanking and HDFC Bank - Third Party Transfer Setup Form
You can also pay for your Mutual Fund purchases using AUTO-PAY by setting up your bank mandate with us.