This is a false positive from your antivirus.

Because Alt is a newly released program, some antivirus software (McAfee, Windows Defender, etc.) may flag it as an unknown program.

Please follow the steps for your security software:

Windows Defender

If you see a "Windows protected your PC" screen, click More info → Run anyway.

McAfee

Open McAfee and find the Alt installer in Quarantine or Threat List.
Select it and click Allow or Restore.

Chrome

Go to chrome://settings/security → Change "Enhanced protection" to Standard protection.
Re-download and install, then restore Enhanced protection.

Other antivirus

Add the Alt installer (.exe) to your antivirus whitelist/exceptions, or temporarily disable real-time protection during installation, then re-enable it.

After installation is complete, fully quit Alt once and restart it.

Dewitte et al, International Journal of Behavioral Development, 2021