15 char exactly

I All, i spent hours to search some solutions...but still no understand syntax :-( and have great problem with my GPU

I' ve a radeon RX 460 4 gb but with -d 2 parameter hashcat not work.....

and so on....

I need to crack a 15 exactly char password of wifi. only lower case and number (important fix 15 char not less not more)
 
I write mask -1 ?l?l..... 15 times and -2 ?d?d?....15 times... not work

I read wiki but can undestand method :-(

What is correct syntax? I run on windows....

thanks

MAX