Here is a script that automatically converts input to upper case:
function onEdit(e) {
if (typeof e.value != 'object') {
e.range.setValue(e.value.toUpperCase());
}
}
From the spreadsheet, go Tools > Script Editor. Put the above code in the code window (replacing anything pre-filled there), and save.
If you wanted to enforce upper case and single-character entries, then data validation could be used, with custom formula
=regexmatch(A1,"^[A-Z]$")
This is assuming the upper left corner of the range is A1, and that you expect letters of Latin alphabet. The regular expression ^[A-Z]$
means: beginning of string; single character in the range A-Z; end of string.