In Ada the not equal operator is /=.
At times, you use the SQL not equal operator (<>) or the C/C++ not equal operator (!=). These are recognized by gnat and it clearly tells you what the problem is. Replace the operator with /= and it will compile.
This is an error because the text of your program is invalid if you don't use the correct not equal operator even if others are clearly understood and would not otherwise make the syntax incorrect.