When you use Microsoft Word to save a document, a security certificate error usually means that Word cannot verify the source of the document or the integrity of the content. This may be caused by the following reasons:

1. Expired or invalid certificate: If the digital signature certificate of the document has expired or been revoked, or the issuing authority of the certificate is not trusted, then Word will not be able to Verify the source of the document. This is to ensure the security and integrity of the document and prevent malware or tampering with the document.

2. The document comes from an untrusted source: If the document comes from an unknown or untrusted source, Word will warn you of possible risks. This can happen to documents via email or downloaded from the internet. For security reasons, Word will give security warnings to prevent opening potentially malicious files.

3. The security level is set too high: If you set the security level of Word very high or modify the default settings, you may encounter security certificate errors when saving the document. Higher security levels increase verification of document origin and content but may interfere with normal operations.

Before solving this problem, you can try the following methods:

1. Check the certificate: Make sure your digital signature certificate is valid and has been issued to you or your Trusted institutions. If the certificate has expired or is invalid, you may need to contact the certification authority to renew or reissue it.

2. Trust the source of the document: If you trust the source of the document, you can choose to ignore the security warning and open the document. Note, however, that you should only take this action if you are very confident in the source and content of the document.

3. Adjust the security level: If you have modified the security level settings of Word, please try to restore it to the default settings, and then try to save the document again. Security level settings can be found in Word's Options or Security Center.

If the above method does not solve the problem, I recommend that you contact Microsoft support or consult a professional for further assistance. They will be able to provide more detailed guidance based on specific circumstances and error messages.