Hi Jacyln - I strongly recommend to go through the basic Mendix courses before you post these types of issues.
In your case, your app references the previous field Subtask_Description somewhere. I can’t see it on your screen because your screenshot cuts exactly that piece of information off on the right side.
Go to where that place is that references the Task.Subtask_Description field, double click on it and change it to Subtask.Subtask_Description.
Jaclyn, use Mendix Studio Pro 8.17.0 (desktop) instead of Mendix Studio (webversion). You are using Mendix Studio, the web version. The webversion is currently only for use by what is known as ‘Citizen developers’ and it has only very limited functions.
Download and install Mendix Studio Pro, https://marketplace.mendix.com/link/studiopro/, Click on tab ‘older releases’ and install 8.17.0, as version 9.0.2 is a buggy beta (9.1.0 will be released in a couple of days/weeks).
If your app’s version is 9.0.2, you will need to start fresh to get to an app in version 8.17.0.
Once you have the desktop version, you will see that the errormessages are clear and you will succeed in getting rid of the error.