Adobe does not allow for encrypted pdfs (ie., ones with a password) to be merged without removing the password security first. This is an industry standard.
Password protected files
You may unwittingly have pdfs that are password protected. These are most commonly ones that come from a product manufacturer or a customer invoice. If you know the password of the file you can easily remove the security on the file which will allow it to be merged into a final document. However if the file is not your original file and is password protected you may have to request a file that doesn’t have password protection.
Whichever method you use make sure that the file you have in simPRO is now unlocked and then re-import the job into Dokkit.