This error often appears either when the related record for an email doesn't match the expected merge fields OR when the merge field itself is not what the Lightning composer expects.
Ensure that the "Related To" field in the email composer has the expected record and that all merge fields in the email reference the same object.
This error can also occur when the merge fields are not in the Handlebar merge language (HML). For example, instead of using {!Object.X}, use the {{{Object.X}}} format.
For Contacts and Leads, remember to use the new Recipient merge fields not Contact or Lead specific merge fields for addresses.
If you still experience the error after checking the above scenarios, please open a case with Support to troubleshoot further.
See also:
Use Merge Fields in Email Templates and Letterheads
Considerations for Using Merge Fields in Email Templates and Letterheads
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