"How do I fix this problem?"
In your case, you need to go back to the Income section and enter the entire amount of capital gains as "Capital Gains Attributable to Income" (see screenshot). Thereafter, you will have to distribute all income (in the Distribution section) to your beneficiaries.
Note that the type of trust will invariably change to Complex, so you will then have to go back to the beginning of the interview and specifically select Simple Trust.
