The answer is in the manual under the specifications for the LOOKUP...
Array variables with a variable index (ex. htxt[x] ) may not be used in LOOKUP although array variables with a constant index (ex. htxt[1] ) are allowed
So, don't use what doesn't work, and use what does work and change it around a bit...
Code:
alpha2:
gosub getkey
LOOKUP key,[" KLMNOPQRST"],tempx
htxt[x] = tempx
htxt[x] = htxt[x] + 1
RETURN
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