。
Not everyone can write well.
Literal
Anyone [topic-も-が] masterpiece [object-を] can-write reason not-is.
誰もが…わけではない ('it's not the case that everyone...') is a partial negation pattern — not 'nobody can' but 'not everyone can.' 名文 (めいぶん, 'masterpiece of writing, well-written prose') is the literary term. 書ける is the potential of 書く. わけではない is the 'it's not that' correction pattern. A thoughtful observation about writing ability that demonstrates an important logical distinction in Japanese.