Problem is the unit cm which you add at the end.
If "Len" is larger than 2 cm your program returns the string "n/a" and you can't add a unit to a string. You can add a unit to a scalar numeric value or to a matrix of numeric values, but not to a string - hence the error message.
BTW, inline evaluation may work OK most of the time but sometimes can lead to unexpected effects. Therefore, I have made it a habit to always separate the definition of a variable and its evaluation/display for safety reasons, as shown in the image above. This also shows clearly that it is not the program that causes the error, but the attempt to display a string with a unit.
Solution depends on what you intend to do with a variable which sometimes represents a length and sometimes a string.
For example you may replace the string by the built-in constant "NaN" ("Not-a-Number", -> About the NaN (Not a Number) Built-In Constant) or even better for consistency you may add a length unit to NaN, too.
If you need later to programmatically ask if "y" is a numeric quantity or a NaN you may use the boolean function "IsNaN()" because something like "if y=NaN ..." does not work.
Every calculation using y when its NaN will result in NaN, too. Demonstrated below via the simple "Area" function:
The "return" statement is not necessary and also the creation of a program structure (the double bar at the left) could be omitted.
Calculation of "z" shows that you also could use the if-function (similar to the usual spreadsheet "if") instead of the programmed if-statement.
Prime 11 sheet attached
