Did you upload the files using Plesk’s File Manager, or another method?
Do you have SSH access? If yes, have you tried this tip from the Troubleshooting page?
osTicket Awesome uses standard permissions.
All folders should be 755 (drwxr-xr-x) and all files should be 644 (-rw-r–r–).
If you suspect that your permissions are messed up you can easily reset them.
For directories
# find . -type d -print0 | xargs -0 chmod 0755
For files
# find . -type f -print0 | xargs -0 chmod 0644