You are using budgeting and active availability control.

Availability control is a function that monitors whether the budget for a project is sufficient for planned and actual costs. Availability control can be activated in different ways depending on the settings in the availability control profile and the tolerance limits. Some of these ways are:

Automatic activation with budget assignment: Availability control is activated when you assign budget to a WBS element or a network activity.

Automatic activation with the first actual posting: Availability control is activated when you post the first actual cost to a WBS element or a network activity. Reference: https://help.sap.com/docs/SAP_S4HANA_ON-PREMISE/4dd8cb7b1c484b4b93af84d00f60fdb8/79ceb65334