You have the following possibilities: select the English keyboard and press the key between ESC and TAB, select the English keyboard and press the key above the right SHIFT, type shortcut key ALT+039, type shortcut key ALT+096. Notice: If you write an apostrophe like inverted commas (2 commas at the top) or if you make spaces around the apostrophe, it will be evaluated as a mistake. If you write an apostrophe correctly and the sentence is still evaluated as wrong, the problem will probably be in something else, such as in missing punctuation.