This is apparently a direct rollover that occurred in 2018. A rollover from a 401(a) to an IRA is always reportable and the 401(a) plan will be issuing a Form 1099-R. The Form 1099-R will have code G for a direct rollover from a traditional 401(a) account or code H for a direct rollover from a Roth 401(k) account. The distribution from the 401(a) must appear on your tax return. If it's nontaxable, the gross amount is to be included on Form 1040 line 4a but will be excluded from line 4b and the word ROLLOVER is to be included next to the line.
If you already filed your 2018 tax return but failed to include this nontaxable rollover, you probably don't need to amend your 2018 tax return but you need to be ready with supporting documentation in case the IRS questions whether or not you completed the rollover. Without that, the IRS can assume that the rollover was not completed and can assess taxes on the 401(a) distribution, should they choose to do so.