Hello!
I would try not to panic about a few spelling mistakes (easier said than done, I know!). Of course, firms expect a reasonable level of attention to detail, but assessment centres are usually assessing a much broader range of things rather than whether each word is perfectly spelled under pressure.
One or two spelling mistakes will probably be absolutely fine, especially in timed conditions, as long as it is not something consistent throughout the work or to the point where it affects readability. There is a big difference between a couple of minor types and work that comes across as generally careless!
I would try not to overanalyse after the fact - I've definitely spotted typos or ran out of time in written exercises before and thought that I had no chance, but still progressed and got offers!