will not let me digitally sign

Hello ​@Tanya 

I hope you are well, and thanks for reaching out. Sorry for the trouble you had.

Could you please share more details about the issue?

• Are you using Adobe Acrobat Reader or Acrobat Pro, and what version (Menu > Help > About Acrobat)?
• Has this same document been reused year over year, or was a fresh copy created this year before collecting signatures?
• Do you know if anyone edited the document after the first signature was applied (even filling fields, saving, or printing to PDF)?
• Are you trying to add your signature after all staff have signed, or are signatures being added in multiple rounds?

Please note that both screenshots and the messages tell the reason for the error: 

This is a Reader-enabled form. When it was first created, someone used Acrobat Pro to add extended rights so your staff could sign it in the free Acrobat Reader. A hidden signature over the whole file protects those extended rights. Each time the form is signed and saved across your nine revisions, the file changes, and at some point, that change breaks the seal on the extended rights.

That is exactly what the second message is telling you: the extended features are no longer available because the document has changed since it was created. Once that seal is broken, Reader stops letting anyone add another signature, and it also stops validating the existing ones, which is the "At least one signature is invalid" you see in the first screenshot.

Suggestions:

• Sign in Acrobat Pro instead of Reader, if you have it. Acrobat Pro does not rely on the Reader extended rights and does not throw this error, so you can usually add your signature there directly. 
• If you only have the free Reader, the form needs to be re-enabled. Whoever created the original form opens the current signed file in Acrobat Pro and applies File > Save as Other > Acrobat Reader Extended PDF > Enable More Tools (Includes Form Fill-in and Save), then sends it back to you to sign in Reader. Note that re-enabling after this many changes can be unreliable on a heavily revised file.
• About the invalid-signature flags: a broken Reader-rights seal often shows the signatures as invalid in Reader even when the people did sign correctly. Opening the file in Acrobat Pro and running Validate All usually gives a truer picture of which signatures are genuinely fine versus flagged only because the rights seal broke.

For more details, you can refer to these articles:

https://helpx.adobe.com/acrobat/desktop/e-sign-documents/manage-digital-signatures/validate-digital-sign.html

https://helpx.adobe.com/acrobat/desktop/work-with-pdf-forms/create-forms/allow-form-saving.html

Let us know how it goes, and share your findings.

Regards,

Anand Sri.