Can you get a waiver on repaying ACA premium tax credits if you did not know you were going to be approved for SSDI and received a lump sum payment?

Unfortunately, there is no waiver currently.  However, the repayment of ACA premiums is limited.  For example, a single individual with income between $29,160 and $43,740 would have to repay no more than $950 if they received too much federal premium tax credit.  This should be calculated for you on Form 8962.

There's a couple things you can do to reduce your MAGI for calculating your repayment amount. Here's a link showing the FPL Amounts. 

• Make sure that the 1095-A you entered has an amount in Column B.  If not, use this Marketplace Tool to determine what that amount should be. 
• Make an IRA or HSA contribution.  You can test the amounts in the IRA Contribution section under Deductions & Credits to see how much of a contribution would be needed to accomplish this. 

There have been court cases with situations like yours, but Congress has not yet made lump sum SSDI payments an exception for repayment of the PTC.  Here's more info. 

@eweeks_98