As far as I understand you need a biradical system in singlet spin state. In such case you must use a "broken symmetry" treatment, i.e. unrestricted formalism.
In Gaussian software use keywords such as:
UB3LYP/6-311G* Guess=Mix
In the case that no spin density arises you must restart it from triplet calculations (keyword Guess=(Mix,Read) ).
The discussion below will surely be helpful (DFT methods and basis sets for studying radical reactions) https://www.google.com/amp/s/www.researchgate.net/post/What_are_the_recommended_DFT_methods_to_study_chemical_reactions_involving_free_radicals/amp