Snyk - Github (GH) integration requires specific permissions to be enabled for each repo. If not, you could get the following error message:
Also, when reviewing the settings for the GH integration on Snyk UI (settings 
-> Integrations -> Github -> Edit settings), you could see another error message if there are permission issues:
In order for Snyk to be able to perform opening fix and upgrade pull requests, the permission "Write" or above must be granted to all repositories that are going to / being monitored by Snyk.
Please read the article GitHub integration and under the section Required permissions scope for the GitHub integration, you will find more details about which permissions are required by Snyk to complete the different actions from the UI.
Additionally, refer to the article Access permissions on GitHub and Permission levels for a user account repository to get more details about the How-To in GH.
Once confirmed permissions are correct on Github for all the repositories, create a new personal access token and update it in the Snyk App. Try to create a new Open Fix PR, it should work by then.