Problem with xbasic compiler

You also must state which variables will be sent inside the turbo_block:
if you do this:
10 A=7
20 CALL TURBO ON
30 PRINT A
40 CALL TURBO OFF
RUN
0
OK
The result will be zero because all variables are 0 every time you begin a turbo block, but they remain intact when you leave the turbo block:
10 A=7
20 CALL TURBO ON
30 PRINT A
40 CALL TURBO OFF
50 PRINT A
RUN
0
7
OK

In order to include an external variable's value inside a turbo block you must state it like this:

10 A=7
20 CALL TURBO ON (A)
30 PRINT A
40 CALL TURBO OFF
50 PRINT A
RUN
7
7
OK

To include several variables, just use a coma:
20 CALL TURBO ON (A,B,C)

But B and C must contain a value, otherwise it will give you an error:

10 A=7:B=0:C=0
20 CALL TURBO ON (A,B,C)

For dimensioned arrays it works like this:

10 DIM N$(25):A=7:B=0:C=0
20 CALL TURBO ON (N$(),A,B,C)

Anyway, having the variables duplicated (outside the turbo block and inside the turbo block) is usually a bad idea and it consumes twice the space AND can generate fatal errors and bugs when the CLEAR function is not used correctly, and it should only be used if those variables will be used by other turbo blocks or outside turbo blocks.
The usual approach would be:

10 CALL TURBO ON
20 A=7
30 PRINT A
40 CALL TURBO OFF
RUN
7
OK

Regarding NestorBasic, yes, it has a lot of features that makes your life easier (embeeded music player and sound player for example) but it requieres 128kb of RAM in order to work while Turbobasic/Xbasic/BasicKun only needs 64kb, so you should choose depending on your needs.

Another tip:
Adding DEFINT A-Z at the beginning of your listing speeds up the turbobasic code A LOT (it does not need to be inside the turbo-block).
If you need a variable not to be an integer you can always leave some for that pruppose:
DEFINT A-Y
Z will not be an integer, and you can define it any way you want.

Have you tried writing line 30 as such:

CALL TURBO ON (A$(),C,B)

Oh, okay sorry. It must be too long ago for me.
And if you try to put DIM A$(25) first (right after the DEFINT A-Z)?

You can't include strings defined outside the turboblock, you need to define it inside the turboblock.
Also, take into consideration that the variable names are just 2 characters long, so if you write more than 2, only the first 2 will be taken into account:

-prime will be read as pr
-premature will be read as pr as well

This will cause you many issues. Just use 2 characters names (p1, p2...)

Here you can find how to use xbasic/turbobasic/basicKun and the differences between regular basic and xbasic (it is in spanish but you can google tranlate it):

https://www.konamiman.com/msx/msx2th/kunesp.txt

I found some more (in english):

https://konamiman.github.io/MSX2-Technical-Handbook/md/KunBA...

Just google:
-msx xbasic manual
-msx turbobasic manual
-msx basic kun manual

You will find plenty of stuff.

thanks a lot!!!