Hey @44011406 welcome to the forums!
The Run button no longer exists. Due to recent changes you will have to Fork unless the creator Deploys the Repl, in which case you will have to get the link to the deployed program. These changes apply to normal console based programs like Python, C, etc. too, but there is no way to deploy them unless you set up some kind of server to run it (like Flask for Python) and use an Autoscale Deployment.
The error that you experience when forking is probably because the repl is a proxy that bypasses restrictions and is against ToS.
Hope this helps!