the meaning of the underlined part
The passage below comes from More Word Smart.
https://books.google.co.kr/books?id=GQUm_kVn80AC&pg=PA164&lpg=PA164&dq=%22any+canvas+that+didn%27t+have+numbers+printed+on+it%22&source=bl&ots=lsfj_FGllL&sig=ITQs0lo3otI_2u7RC6E3kUqeCW8&hl=en&sa=X&ved=0ahUKEwjxybaByOrJAhUQ2WMKHeF9BrgQ6AEIHjAB#v=onepage&q=%22any%20canvas%20that%20didn%27t%20have%20numbers%20printed%20on%20it%22&f=falseMiriam considered herself an excellent painter, but she was distinctly a layman; she couldn't make much headway on any canvas that didn't have numbers printed on it. In this sentence I can't grasp the meaning of the underlined part.
It's not hard to read it literally.
But I can't make out the meaning in context of the underlined part.
First, I'm not 100% sure about what 'it' refers to.
I think 'it' refers to 'the canvas'.
(Am I right?)
Then, what is the canvas that didn't have numbers printed on it?
I know this question is not well-composed.
I make an awful question thread whenever I can't not figure out exactly what I don't know.
This is just the time.
But this is the time when I need your help the most.
Regards.