I think you will not find a solution by putting something else in ‘choices’ and I assume it’s not working because the exemple you followed is not for multiple select field. So you probably try to put an array somewhere a string is required.
Use print_r to display your get_field result and find how to use it.
If you don’t find the solution let’s us see your code because is not easy to find what’s exactly goes wrong with no code lines.
Hope it’s help.
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