Step 11 of 18 of the SMILES tutorial. Some aromatic nitrogens do carry a hydrogen, and SMILES cannot guess which ones. Write those as a bracket atom with an explicit hydrogen count: [nH]. So c1cc[nH]c1 is pyrrole, whose N–H points out of the ring while its lone pair completes the aromatic sextet. Compare with the previous step: a bare n means no hydrogen, [nH] means exactly one — mixing them up is the single most common cause of "cannot kekulize" errors.
It opens on c1cc[nH]c1, which you can edit in place: the structure follows every keystroke, and so does the notation when you draw instead.
Convert between chemical structures and SMILES in both directions, search a list of structures by substructure or SMARTS, and learn the notation step by step. Everything runs in your browser — no structure you open ever leaves your machine — so the tool needs JavaScript.