This type of error usually occurs if you have a firewall/security plugin installed that blocks the REST API endpoint of Fluent CRM.
If you have any such firewall/security plugin, you may whitelist the Fluent CRM endpoints. Additionally, please check the server-side firewall. We often find that server-side firewalls, like Mod-security, block the REST API endpoints of Fluent CRM. In such cases, you can contact the hosting provider to whitelist the Fluent CRM endpoint.
Could you please check the server-side firewall?