To create purchase requisitions for the materials used in your project, you must ensure that the materials have a procurement type that allows external procurement and a material number that identifies them uniquely. Some of the possible reasons why no purchase requisitions have been created after releasing the project are:
The procurement type in the material master is set to in-house production (E). This means that the material is produced internally and not procured externally from a vendor.
The material has no material number (N). This means that the material is a non-stock item that is not managed in inventory and has no master record. Non-stock items require manual creation of purchase requisitions. Reference: https://help.sap.com/docs/SAP_S4HANA_ON-PREMISE/4dd8cb7b1c484b4b93af84d00f60fdb8/79ceb65334e6b54ce10000000a174cb4.html?&version=1909.002

To carry out the internal project work as soon as possible, you have to maintain the control key of an activity in capacity requirements planning. Capacity requirements planning is a process that determines the capacity requirements and availability for work centers or resources that are used to execute project activities. The control key is a parameter that defines the basic properties of an activity, such as whether it is internally or externally processed, whether it requires confirmation or not, and whether it is relevant for scheduling or not. The control key also determines whether an activity is relevant for capacity requirements planning or not. To carry out the internal project work as soon as possible, you have to use a control key that activates capacity requirements planning for the internal activities. Reference: https://help.sap.com/doc/0f069b2e08b748f1b480d4c8b46c3c49/100/en-US/6a7a9f0a6e421014e10000000a174cb4.html

A WBS element is an object that is used to allocate the costs from a claim to the project structure. A claim is a document that records a customer's request for compensation due to deviations from the agreed terms and conditions of a project. A WBS element is a structural element of a project that represents a task or a partial task. You can assign claims to WBS elements and post costs or revenues to them. Reference: https://help.sap.com/doc/0f069b2e08b748f1b480d4c8b46c3c49/100/en-US/6a7a9f0a6e421014e10000000a174cb4.html