When I use Fill In Sign Mode on Adobe Acrobat Reade DC, ma on Windows 10, the toolbar does not come up where I could make checkmarks, make circles, or add text.
Sorry for the delay in response to your query.
What is the exact version of Adobe Reader DC you are working on? Open Adobe Reader>Help>About Adobe Reader
Adobe Acrobat official update (19.008.20074) is now available. This update will be automatically pushed to all existing installations of Acrobat. If you want, you may also manually trigger the update early by opening the application and going to Help > Check for Update
If it still doesn't give any luck, then you may use the cleaner tool to uninstall the existing version Download Adobe Reader and Acrobat Cleaner Tool - Adobe Labs of Reader and reinstall it through- Adobe - Adobe Acrobat Reader DC Distribution
Hope this helps.