Excel error HRESULT: 0x800A03EC while trying to get range with cell's name

The error code 0x800A03EC (or -2146827284) means NAME_NOT_FOUND; in other words, you've asked for something, and Excel can't find it.

This is a generic code, which can apply to lots of things it can't find e.g. using properties which aren't valid at that time like PivotItem.SourceNameStandard throws this when a PivotItem doesn't have a filter applied. Worksheets["BLAHBLAH"] throws this, when the sheet doesn't exist etc. In general, you are asking for something with a specific name and it doesn't exist. As for why, that will taking some digging on your part.

Check your sheet definitely does have the Range you are asking for, or that the .CellName is definitely giving back the name of the range you are asking for.

I ran into this error because I was attempting to write a string to a cell which started with an "=".

The solution was to put an "'" (apostrophe) before the equals sign, which is a way to tell excel that you're not, in fact, trying to write a formula, and just want to print the equals sign.

I got the error with a space in a Sheet Name:

using (var range = _excelApp.Range["Sheet Name Had Space!$A$1"].WithComCleanup())

I fixed it by putting single quotes around Sheet Names with spaces:

using (var range = _excelApp.Range["'Sheet Name Had Space'!$A$1"].WithComCleanup())