I keep getting this error trying to finish my 2018 amendment: "Form 8965 - health coverage exemption: your calculations could not be completed. reference code: 3-121-570"

You don't. 1095-C is purely for your own information. It is not added to the tax return. The only thing you need to enter in the Healthcare section is the number of months you were covered. If you have a 1095-A in addition to the other form, you will need to add that as well.