Why is this an issue?

Passing arguments of the wrong type to a call to System.SysUtils.Format raises a runtime exception. In particular:

%d, %u, and %x must be an integer value
%e, %f, %g, %n, and %m must be a floating-point value
%p must be a pointer value
%s must be a string, character, PChar, static array of Char, or variant value
Floating-point width and precision must be integer values

How to fix it

Ensure that the arguments to the Format call match the specifiers in the format string:

Format('%d got %.2f percent on the test.', ['Bob', 74.599]);

Format('%s got %.2f percent on the test.', ['Bob', 74.599]);