Remember, that the if condition must be a scalar. You provide a comparison of two vectors:
if data.price(1:end-1)<data.price(2:end)
Then Matlab executes this internally:
if all(data.price(1:end-1) < data.price(2:end)) && ...
~isempty(data.price(1:end-1) < data.price(2:end))
I assume you want something else.
Either use logical indexing. Or create a loop over all "trades" like:
for iTrade = 1:numel(data.price) - 1
if data.price(iTrade) < data.price(iTrade + 1)
...
end
end