Please check the version of Salesforce that you are using. Salesforce has restricted API access, but there's no limitation in Freshdesk as such. So you’ll be able to see all Freshdesk data in Salesforce, but we will not be able to pull Salesforce data into Freshdesk.
REST API is available by default only on Developer, Enterprise, Unlimited, and Performance versions.
*Professional can have API Temporarily turned on by contacting your Account Executive.
Please review the article: Enabling API (https://help.salesforce.com/apex/HTViewSolution?urlname=Enabling-API&language=en_US)
Salesforce app can contain business logic such as classes, triggers, email services, etc. written in Apex and as a general rule, Apex is not supported in GE (General Edition)/PE (Professional Edition), so it will not run in these editions. Hence, any app that is based on APEX will usually give an error when installed from the Salesforce Marketplace.