System exception.Object reference not set to an instance of an object. at Source: Invoke System1_Upload_YearlyReport workflow: Assign out_UploadID",

For some of the items it worked very well, but why is it showing me errors now?

Hi @Anky,
Please check mentioned InvokeWorkflow Files and their Argument if all have the proper values. This error means that variable/argument don’t have any reference (is not assigned anywhere).

In the flow, check the right side expression of the out_UploadID.
One of the variables are not set.
Check if you are getting a value in it by using a write line/ check if the get of the upload id is returning a value.