Stock items can be procured using planned orders, third-party orders, or purchase requisitions and reservations for WBS elements. Preliminary purchase requisitions are used for non-stock items and reservations are used for stock items that are already available in the warehouse. Reference: https://help.sap.com/doc/0f069b2e08b748f1b480d4c8b46c3c49/100/en-US/6a7a9f0a6e421014e10000000a174cb4.html

One possible reason for the error message "Business transaction cannot be carried out" when releasing a network is missing authorizations to release material components. Material components are materials that are required for executing a network activity. To release material components, you need to have the authorization object C_AFKO_ATY with the activity 02 (change) for the order type of the network. Reference: https://answers.sap.com/questions/7530952/the-business-transaction-cannot-be-carried-out-mes.html

To confirm the working hours of different work centers for a single activity, you can create multiple confirmations for the activity and use the appropriate work center for each confirmation. A confirmation is a function that allows you to record actual data for activities and activity elements, such as actual dates, actual durations, actual work, and so on. A work center is a parameter that defines where an activity is performed or who performs it. A work center can have various attributes, such as capacity, cost center, formulas, and so on. To confirm the working hours of different work centers for a single activity, you can create multiple confirmations for the activity and use the appropriate work center for each confirmation. This allows you to capture the actual work done by each work center separately and update their capacity requirements and costs accordingly. Reference: https://help.sap.com/doc/0f069b2e08b748f1b480d4c8b46c3c49/100/en-US/f373bb53707db44ce10000000a174cb4.html

The system can activate requirements (such as reservations or purchase requisitions) in different ways, depending on the settings in customizing and master data. Some of these ways are:
Immediately, by saving the project: This means that the system creates requirements for all materials assigned to WBS elements or activities as soon as you save the project. This option requires that you set the indicator "Requirements immediately" in customizing for your project profile.
When releasing a network or an activity: This means that the system creates requirements for all materials assigned to a network or an activity when you release them. This option requires that you set the indicator "Requirements on release" in customizing for your network type or activity type. Reference: https://help.sap.com/docs/SAP_S4HANA_ON-PREMISE/4dd8cb7b1c484b4b93af84d00f60fdb8/79ceb65334e6b54ce10000000a174cb4.html?&version=1909.002