Am 74, Social Security is sole income. Fedloan requires a tax return each year to qualify for Income Based Repayment. Why can I no longer file my 2018 return online?

Without taxable income, it is a blank form.  You cannot transmit a blank form. You may want to contact Dept of Ed to see if there is another option to prove income when you do not need to file a tax return.

Your other option is to print and mail the 1040 form.  

https://studentaid.ed.gov/sa/repay-loans/understand/plans/income-driven/questions