。
She needs to be more careful.
Literal
[topic-は] more careful [adverbial-く] do necessity [subject-が] exists.
~必要がある ('it is necessary to') is a standard construction for expressing need or obligation. 注意深い ('careful, attentive') is an i-adjective compound: 注意 ('caution') + 深い ('deep') — 'deep in caution.' Its adverbial form 注意深く modifies する ('do, act') — 'act carefully.' The whole construction もっと注意深くする ('act more carefully') describes the recommended behavior, with 必要がある packaging it as a necessity.