In Japanese you can add -やすい or -にくい to a verb stem in order to say if something is easy or hard. For example with 書きます if you remove the ending you have 書き. 書き＋やすい＝書きやすい(Easy to write) 書き＋にくい＝書きにくい(Hard to write) Another example: 食べます、食べ＋にくい＝食べにくい(Hard to eat) With your sentence it is saying もっと書きにくいペン so it is asking for a pen that is "more easy to write (with)" or "EASIER to write (with) "

This 易い (yasui) means easy and this 安い (yasui) means cheap. "Please use a pen that is easier to write with"

a pen thats easier to write with