im using iMac and some of letterhead missing

Try these checks in Word for Mac:

1. In Word, go to View > Print Layout. Images and logos may not display correctly in Draft, Outline, or Web view.
2. Go to Word > Preferences > View and make sure Drawings is enabled. This controls whether images and shapes are shown on screen.
3. Go to Word > Preferences > Print and make sure Drawing objects is enabled.
4. Test whether the problem happens only in one document:
   • Create a new Word document.
   • Insert the logo image.
   • Save, close, and reopen it.
   • If the image shows correctly there, the original file may be the issue.
5. If the document was created on another computer, the file may be using substituted fonts or older rendering behavior. In that case, install missing fonts on the Mac and keep Office updated.
6. In any Office app, go to Help > Check for Updates, enable Automatically keep Microsoft apps up to date, then install all available updates.

If the missing part is related to the header image itself, and text is placed over the logo, set the image layout to Behind the Text.

References:

• Word documents - images and graphics missing - Microsoft Q&A
• My fonts have changed
• Office documents are not rendering as expected
• Error: [Filename] uses a file type that is blocked from opening in this version
• In Word, my header is an image and when I paste letters on it - the image disappears. How can I fix this? - Microsoft Q&A