。
She made many spelling mistakes.
Literal
She [topic-は] a-lot [genitive-の] spelling-character [genitive-の] errors [object-を] did.
つづり字 (tsuzuriji, 'spelling letter / spelled letter') refers to the sequence of characters used to spell a word — a slightly old-fashioned or technical term, more common in older textbooks than in current usage. Modern Japanese more often says スペルミス (loanword 'spell-miss') or 綴りの間違い for 'spelling mistakes,' especially in alphabetic-language contexts; つづり字 carries a slightly classroom-grammar feel. ~をする with a noun like 誤り ('mistake') gives the standard light-verb construction for 'make a mistake' — paralleling 間違いをする and ミスをする.