Specflow generates nunit tests from the gerkhin feature files by generating a code behind file. This process is often quite sketchy and sometimes creates broken files. If you delete the .cs file with the same name as the .feature file it should get regenerated.
Be aware that specflow doesn't work on dotnet core , only framework.
It doesn't work particularly well in anything that isn't visual studio either.
I have often spent hours chasing whitespace around to ensure that the regexes that are used to match the steps work correctly. If you are having trouble with these I would recommend pasting them into regex101.com with the subroutine names, this site will explain what the regex matches in the context of the example data you give it.
Hopefully we can all move on from this daft idea of BDD tests that are secretly nunit tests, who are these non-technical people that this is for and why are they working on technical projects ?