U+(301) or U+(0301) in Unicode is a combining acute accent. It is a stress mark equivalent to U(0027) an apostrophe, or U(00B4) an acute accent, amongst others. You can also insert it directly, or test whether this font contains it by using INSERT > SPECIAL CHARACTERS > COMBINING DIACRITICAL MARKS >
However, the font you are using must contain this character. If not it will not show up. In this case you can try an apostrophe or directly accented character.
You don’t say what operating system you are using, the language setting, keyboard and version of LibreOffice and font, all of which effect an answer. Also what characters you are trying to enter.
LibreOffice uses Unicode which one can enter directly using Linux and the combination CTRL/SHIFT U(+the code, in this case 0301or 301). For example U+0301 with A H a (blank) gives Á h́ á ́.
However, the result does depend on the position on the line, for example when placed as the first character on a line you may get a faint positioning circle as a combining diacritic by definition will not be the first character, it will always be placed on top of another character.
Hope this helps…Peter