認定証が求められる最も多いケースは商品の引渡し時なので、営業部がダウンロード権限を持つのがよい。と言いたいです。
Since certificates are most required at handing over timing, it is better to give right of download certificate to Sales dept.
Since certificates are commonly required at the time of delivery of goods, it is better to give the sales department the right to download the certificate.
ouchiさんの英文を直しました。下記の通りになります。
「Since certificates are commonly required at the time of delivery of goods, it is better to give the sales department the right to download the certificate.」
変更前 変更後
「most required」 → 「commonly」
「handing over time」 → 「the time of delivery」
「right of download certificate to sales dept.」→「the sales department the right to download the certificate.」
この英文の方が自然なので、よかったら使ってみてください。
ご参考になれば幸いです。
"Since certificates are most often requested at the time of product delivery, it makes sense for the Sales department to have download access."
"Since certificates are most often requested at the time of product delivery, it makes sense for the Sales department to have download access."
"The most common time certificates are required is when we deliver the product, so the Sales team should have permission to download them."
"Because certificates are usually needed when handing over the product, it’s better for Sales to have the authority to download them."
「最も多いケース」は most common case / most common time が自然です。
most required は少し不自然で、「要求される・求められる」は英語だと be requested がいちばんよく合います。
required も使えますが、「規則で必須」のニュアンスが強くなります。会話や社内文書では requested の方が無難です。
アメリカ合衆国 
