cannot open ringcentral. It says another softphone is running on my computer

This may be because the RC app might be running in other system. Currently only 1 RC app can be run at a time and is registered with the device because it uses Direct lines for communications.

To resolve this you need to sign out the app, Deauthorize the app and then use other app to log in.

Check the reference to resolve:

https://support.ringcentral.com/article/4219.html