Salesforce field name not visible when mapping field in Account Engagement

A Salesforce field can only be mapped to one Account Engagement field at a time. If the Salesforce field is at the account, check under Admin > Configure fields > Account fields (or in Lightning: Account Engagement Settings > Object and Field Configuration > Account Fields) to see if the field is already mapped to an existing Account Engagement field. 

Tip: Search for the API name of the Salesforce field, as that is how the field will appear under the "SALESFORCE.COM FIELD NAME" column. For example, the API name of a Salesforce field may look something like "Custom_field_c".
Use (CTRL + F) or (Command + F) on your keyboard to more quickly search for this field name on the page. 

For a Salesforce opportunity field, check under Admin > Configure fields > Opportunity fields in Classic or Account Engagement Settings > Object and Field Configuration > Opportunity Fields in Lightning. 

For a Salesforce lead or contact field, check under Admin > Configure fields > Prospect fields in Classic or Account Engagement Settings > Object and Field Configuration > Prospect Fields in Lightning. 

If this Salesforce field is mapped to the incorrect Account Engagement field or the user would like to remap it, edit the field in Account Engagement > under the dropdown "salesforce.com field name" select the 'blank' value at the top of dropdown > save the field. 

This Salesforce field is now available for use and can be mapped to a new or different Account Engagement field. 

NOTE: If you have multiple Business Units and the Salesforce field is already mapped in one of the business units, it will not appear in the dropdown list of salesforce.com field names.

The connector user's profile needs access to this field in Salesforce and it should not be 'hidden'.

* 'View Field Accessibility' is where you'd see if it's hidden or editable.
* 'Set Field-Level Security' is where you set it to 'visible' or 'read-only' or neither.

You want to be sure that your connector user's profile has visibility into this field and it is not 'hidden'.
 

The Account Engagement metadata may not have refreshed between the time you created the Salesforce field and tried to map the field in Account Engagement. To help kickstart a manual refresh, edit the Account Engagement field and click on the arrow icons to the right of the "salesforce.com field name". This forces a backend data refresh from Salesforce and gets that new Salesforce field to show up in Account Engagement for your immediate use. 

If you have an Account Engagement account with the Advanced Edition, check your Marketing Data Sharing rules to ensure the field you are looking to map is not in use. If it is in use, it cannot be mapped with Account Engagement.

You can check your Marketing Data Sharing rules in the Account Engagement Lightning App only under Account Engagement Settings > Connectors > Edit settings for the salesforce.com connector > Marketing data sharing tab.
You will find the rule for each object with the API name of the field in use.

If you have multiple business units, use the business unit switcher and repeat these steps to check the Marketing data sharing rules in each business unit.