Why is turbotax rounding up the amount that I enter for the advanced Child Tax Credit payments? My amount received ended in .50, but turbotax rounds that 50 cents up.

Outsmart the software.   Round one up by a dollar and round one down by a dollar so it comes out even.  Example:   10.50---round up to 11   10.50  round down to 10    you get 21 when they add together and it keeps the software happy.   And all you need is the real total to keep the IRS happy.