How to answer why I left a previous job?

I have an interview tomorrow and am trying to work out how to answer this potential question.

I left the job in March, and the real reason is that I made an error and essentially had to resign. Nothing dangerous or anything, just against regulations. It's definitely gone down as a resignation not a firing so that's what my reference will say. I then had a temporary job from May - August, so as it's not even my most recent job, they may not ask, but they still might as who the hell leaves a job just as COVID was starting..

I was thinking of just saying I received a new offer, so I quit but it then fell through due to the pandemic. Is that believable? A barefaced lie admittedly but I need a job.

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