The file wasn't opened successfully as the error says...use the alternate return to find out additional information on why...
[fid,msg]=fopen('ARBY STARL 170310-200514 LT Edited_OBS TIDES 6mins.dat','r','n','ARAB')
and see what the error message is for hints. I note the character code is not in the listed options so while Matlab will not error it will warn and may or may not provide a correct interpretation. I'd suggest unless there's a known reason, simply use the defaults other than for file permission although the most likely cause for fopen to fail is that the file is not located on the path and so simply isn't found. But, I'd still suggest
[fid,msg]=fopen('ARBY STARL 170310-200514 LT Edited_OBS TIDES 6mins.dat','rt')
as first go and check the returned handle is valid before going on to try to read anything 'cuz "it ain't agonna' work" until it is (valid that is).
