With the latest sources I have downloaded with file epplus-41a0d8d22a57.zip. The following line in Value.cs doesn't compile due to the $ outside the string. I'm not sure if this is supposed to be @ instead. The value.cs file can be found in the relative folder EPPlus\FormulaParsing\Excel\Functions\Text.
if (Regex.IsMatch(val, $"^[\\d]*({Regex.Escape(_groupSeparator)}?[\\d]*)?({Regex.Escape(_decimalSeparator)}[\\d]*)?[ ?% ?]?$"))
if (Regex.IsMatch(val, $"^[\\d]*({Regex.Escape(_groupSeparator)}?[\\d]*)?({Regex.Escape(_decimalSeparator)}[\\d]*)?[ ?% ?]?$"))