You were on the right track!
On line 112, you requested the stock's price, but not all stocks trade every single minute. The algo hit an error when it asked for a stock's price when it didn't have data. To fix it, I added a small guard:
if stock in data:
short_buy=context.portfolio.positions[stock].amount
price=data[stock].price
Also looks like prices and prices_l are identical calculations (is this intentional?) so I made them equivalent.
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