chr$(138)= è
it seems that that uppercase of "è" is not supported in QB64...
and it's pretty weird (although there will be a valid reason for that)
in italian language cannot do without a uppercase of " è " (È)
The base font for Qbasic and QB64 uses the original MS-DOS 8-bit extended ASCII, where all the accented characters, and some Greek characters, and some symbols for drawing blocks, and a few other goodies, take up code 128-255. So yeah, not everything is supported. I always have a table next to me, which shows the basic extended characters.
Otherwise, you have to select some UTF-8 font. There are people here who have become experts in this matter, like Tempodibasic. I'm too lazy, so I just use whatever accents are available in the base font. (Italian uses so few accents that for the most part, they all exist in the base font, other than upper case accented characters. French seems to have been given priority, in this base character set.)