Hi there tomasomi,
It appears that you're having an issue when trying to digitally sign certificates within the Adobe Acrobat Reader on your iMac. The following link shows how to use digital signatures in the Acrobat Reader app: Using digital signatures
If you've followed those steps, you'll also want to make sure that the Acrobat Reader is fully up-to-date. This article from Adobe shows how to possibly do that: Install updates for Acrobat and Reader
If you continue to have issues with Adobe Acrobat Reader, you can reach out to the app developer for further help with their software. The steps below show how to possibly do that:
"Contact the developer of a macOS app
- Open the App Store on your Mac.
- Click the sign-in button
or your photo. - A list of apps appears. Click the app that you want. If you can't find the app, use the search bar.
- The app's product page appears. Click Support or App Support. If you don't see Support or App Support, make sure that you're signed in with the correct Apple ID.
Didn't get the app from the App Store?
If you have an issue with an app on your Mac but you didn't get the app from the App Store, learn how to contact a third-party developer."
Those steps are from this article: How to contact an app developer - Apple Support
Take care!
