1) Debug so you understand where the error is. It can help to split long lines into multiple lines so a single step in the debugger only does as little as possible. I know debugging might seem like an advanced topic you can learn later. It is not. It is the first skill you need to learn. It makes things simpler, not harder.
2) Error messages try to tell you what is wrong. Read them and take time to understand them. Google the error text will often give hints. And if you still can't find out, at least include the error message when you ask for help.
3) What do you expect
int.Parse("SSL/06/XI/2019")
to return? You first need to read the string, then split it up in parts, then increment the part you want, then reassemble the string. Look into the Split() method on strings. There are other (arguable better) ways to do it, but Split() is very simple conceptually.
This also gives you a good place to detect if the previous invoice was from last year so you need to reset the count.
An alternative approach is to store each part of your invoice number in it's own column in the database and assemble the string as it needs to be printed or displayed.