You will need to know your Requirement Activity Type ID. You can find this by the following steps.

1. In a full search, Select requirements
2. Select the type Filter
3. Select one of your requirement types you need the ID for.
4. Click Advanced at the bottom.

This will show you Activities.Type_ID = XXX  Please note the number in the XXX location.

While in Advanced Search, Select Request from under Record Type.

Paste the search below, replacing XXX with the number from the Activities.Type_ID above.

Requests.ID NOT IN (Select Activities.Request_ID FROM Activities WHERE Activities.Type_ID in (XXX))

Click Search.