snap2stamps project.conf only has parameters for the processing.
SNAP specific configuration is handle during SNAP installation, and remains untouched by any snap2stamps process.
I believe you may check whether you have written permission on the folders where SNAP tries to save any file.
Either permission to write in the directory, it may happen if you install it as superuser (sudo) and you try to execute it with another user, or you may not have available space on your disk.
From my understanding, SNAP does not require sudo rights to download or install.
Still, if you use sudo, please after installation remember to give read/write permissions to the user that will use it. Not only the snap directory, but also its hidden .snap directory, where many aux data is downloaded and stored.
My suggestion is: either you are always sudo and you execute the snap2stamps scripts also using sudo command, or you do not use sudo during the installation.
Just try the same instructions but without the sudo command.
Be sure that you are installing it on your user space (where you have permissions to write)
It is difficult if you do not provide the exact error you could get while trying to open snap.
I think you are the only one I have seen so far with such problem with snap not being able to solve it.
I may remember that the snap2stamps works also in windows if you are more familiar with.