[Mauzen]

%s is a format sign, you can only use it in format. https://sampwiki.blast.hk/wiki/Format
So format the string at first, and then use it for fopen:

pawn Код:

new fname[32];
format(fname, 32, "adminjail/%s.txt", sendername);
new File:example = fopen(fname, io_write);