How to create a lisp to insert blocks with AutoLISP?

So, I am kinda new to the AutoLisp world and I wanted to create a lisp that defines a command named "FF" to insert specific blocks. For example, the user types FF, then defines the first point and then types a string like "TFN". The thing is, in this string all characters are blocks, F corresponds to a block, and so do N and T. Here is the code:

(defun c:FF () 
(setq P (getpoint "Define origin:"))
(setvar aux 1)
(setq seq (getstring "Define the sequence of blocks:"))
(foreach str seq
	 (command "insert" str (polar P (* pi 2) (aux*2)))
	 (incf aux)	
)
)

It's supossed to look like this:

By the way, the "aux" variable is used to create separation between the blocks.

Autocad 2010 does recognize the command FF, then requires a point, but when the point is defined it returns this: error: bad argument type: (or stringp symbolp): nil

There are several issues with your code, but the error arises from the use of setvar in the expression:

(setvar aux 1)

Here, the symbol aux is being evaluated as an argument to the setvar function and is yielding a value of nil, which is an invalid string or symbol representing a system variable as required by the setvar function.

Instead, you should assign variables using the setq (aka "set quote"), which will not evaluate the first supplied argument:

(setq aux 1)

You'll also encounter an error with your foreach statement, as seq is a string and foreach can only iterate over lists.

Instead, you'll need to separate the string into a list of individual characters, either using substr or a combination of vl-string->list & chr.

Or alternatively, iterate over the string using substr until you're left with an empty string, for example:

(while (/= "" seq)
    (setq blk (substr seq 1 1)
          seq (substr seq 2)
    )
    (if (or (tblsearch "block" blk) 
            (setq blk (findfile (strcat blk ".dwg")))
        )
        (command "_.-insert" blk "_S" 1.0 "_R" 0.0 "_non" (polar p 0.0 (* aux 2.0)))
    )
)

This will also account for the user pressing <kbd>ENTER</kbd> at the getstring prompt, as the test expression for the while loop will not be satisfied.

Note that incf is not a valid AutoLISP function, unless this is a function that you have separately defined.

You may also want to consider setting the ATTREQ system variable to 0 prior to inserting the blocks, so that you do not receive the prompt to supply attribute values.