Hi @Enes Kavak
To delete an ExpressRoute circuit, it must first be deprovisioned by your service provider. Azure will not allow the circuit to be deleted until the deprovisioning process is completed at the provider level.
After your circuit service provider confirms that the circuit has been deprovisioned, verify in the Azure portal that the Provider status has changed to "Not provisioned".
Only once the status shows "Not provisioned" then only you can proceed to delete the circuit. If the status still shows "Provisioning", you won’t be able to delete the circuit.
Follow the Deprovisioning and deleting an ExpressRoute circuit
I hope this helps to resolve your issue. Please feel free to ask any questions if the solution provided isn't helpful.