Try going to https://beammp.com and getting the SSL certificate from there by clicking the Lock icon on the left of the URL bar. After installing this certificate, your BeamMP should work fine.
Step 1: Click on the circled button
Step 2:
Step 3:
Step 4:
Step 5: Click on the text circled in red this will open a dropdown list
Step 6: Select Base64 encoded ASCII, certificate chain (.pem;.crt)
Step 7: Click Save
Step 8: Find the certificate file you downloaded in File Explorer. Double click the certificate file that you just downloaded
Step 9: Click Install Certificate
Step 10: Select Local Machine, then click Next. Press Yes on the UAC prompt
Step 11:
Step 12: Click Finish
Step 13: Open BeamMP and it should work fine now!