Cannot connect to Wi-Fi after installing VPN program

Unless that VPN is required by your employer or financial institute then it is best removed.

• Private VPN's are Anything But Private and 

• Security Risks: The Dangers of Using Free VPNs – eccu.edu.  

Apps of the types VPN, anti-virus, cleaning and security usually just cause more problems than they prevent. They are unnecessary on the Mac and best avoided.

• XProtect - Protecting against malware in macOS - Apple Support 

• Apple Platform Security - Apple Support