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 manually de-encrypt it. If you are only using the file for information purposes this is perfectly legally however Dokkit as a company cannot provide this as a service.
The fastest way to do this is to use an unlock pdf service (usually free) like SmallPDFs. We do not directly endorse this company or its services and the onus is on you the user to ensure that you are not breaking the original file owners copyright laws.
Whichever method you use make sure that the file you have in simPRO is now unlocked and then re-import the job into Dokkit.