If I have a test in cell A1:
=if(TEST,let cell C1="MESSAGE1",let cell C1="MESSAGE2")

Note the cell with the if-condition in it is not the one that will contain
either MESSAGE1 or MESSAGE2

The reason is to let the user only enter a value in C1 once a value was
entered into B1. MESSAGE1 wold be something to the extent of "First select
the type in B1". This is to overwrite the user's entry into C1.