Maybe property "Disable printing/forwarding/copying to clipboard" is set in form properties
It shouldn't be set to enable copying text to clipboard.
Another reason for not being able to copy field's text could be field property "Hide paragraph when document is Copied to the clipboard" although it has effect in read mode only.
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Other possible reasons are described here. In case your document has a field "$KeepPrivate" then execute this formula on document:
FIELD $KeepPrivate := @DeleteField