1. Log on as an administrator.
2. Navigate to Setup > Administration > Users > Users > Profiles.
3. Select Partner Community User and click Edit.
4. In the Administrative Permissions section, select API Enabled check box.

Note: The preceding workaround is no longer available as Salesforce has disabled the API-enabled permission on all standard external profiles. For more information about disabling the API-enabled permission, see Salesforce Help

Workaround:

For the workaround, perform the following steps.

1. Go to Setup > Administration > Users > Permission Sets.
2. Create a custom permission set.
3. For this custom permission set, go to System Permissions and select the API Enabled check box and save.
4. Assign this custom permission set to the Customer Community Plus and Partner Community users.