I am having trouble accessing our on-prem SharePoint 2016 site through the on-premise gateway. I am able to access other files such as an excel sheet on a local file server but I cannot get it to talk to the SharePoint site. When trying to create a new flow, for example "send a customized email when a new file is added", the site address dropdown shows "No Items." When supplying a custom value, I get the error message "<span>We are unable to find the site address. Please try selecting a site address from the dropdown."
When trying to add a new data source from the sharepoint site using the PowerBI interface, I get this error message:

I have tried the provided troubleshooting guide with no success. I have also tried many different log in credentials formats when creating the connections to the SharePoint site such as "username", "username@domain", "domain\username". I have also shared the gateway with my SharePoint site admin and he received the same error message using his credentials.
I have set up the on-premise data gateway on a local windows server and configured it to run in HTTPS mode. I changed the gateway service account to a managed service account. Network Port Test is successful.
Does anyone know why I would be able to connect to some local data sources using the on-premise gateway but not be able to access the on-prem SharePoint 2016 site?