Want to Merge One Single PDF with mulitple PDF files

Now it's becoming more clear... It's not that there's an error, it's just that you want to do it in batch, for multiple files. You can do that using the Action Wizard. Go to Tools - Action Wizard and click on Create New Action. Then add to it an Insert Pages command from the Pages sub-panel on the left:

Then untick the "Prompt User" check-box and click on Specify Settings:

Select the file you want to insert and then specify where to insert it in the dialog window that opens:

Now add a Save command from the Save & Export sub-panel:

You can click on Specify Settings for this one too, if you want the edited files to be saved under a different name or in a different folder. If you want to overwrite the original files, just leave it as it. Your Action should look like this:

Then click Save and give the Action a name. Now you can run it by clicking it from that list of Actions in the Action Wizard panel. Select the files (or folder) you want to run it on, and click Start.

Wait for it to finish. The last file might remain open in the application (it's a bug of some versions of Acrobat), so close it manually, and you're done!