Please note you need an admin account to fix this.
1. The field-level security of the custom field isn’t set up.
If users don’t have access to a custom field, it won’t appear in the Field settings.
To make it visible: Go to Setup > Object Manager > Select Lead/Contact/Account > Fields & Relationships > Select the custom field not appearing > Make it visible in the Field-Level Security settings.
2. Changes to the custom field settings are affecting data loading.
When you carry out structural changes after you set Fixed value, Fixed value may not reconstruct and custom fields may not appear.
In this case, go to the Scan to Salesforce tab, then the wrench icon, open Admin Settings and click [Clear field settings] to delete the cache.