filename = "smartflux.csv";
That is a basic filename without any directory information.
filepath= 'G:\Shared drives\Project_Crockett\QAQC_plots\EC_Benju\';
Tnat assigns a value to a variable. filepath is not any kind of reserved name in MATLAB, so it does not have any other result: a variable is assigned, nothing else happens.
opts = detectImportOptions(filename,'NumHeaderLines',3); % number of header lines which are to be ignored
You pass the basic filename to detectImportOptions . detectImportOptions is going to search for the file along the MATLAB path but in this case is not going to find it.
What you should be doing is
filepath= 'G:\Shared drives\Project_Crockett\QAQC_plots\EC_Benju\';
filename = fullfile(filepath, "smartflux.csv");
opts = detectImportOptions(filename,'NumHeaderLines',3); % number of header lines which are to be ignored
opts.VariableNamesLine = 2; % row number which has variable names
opts.DataLine = 4; % row number from which the actual data starts
opts.VariableUnitsLine = 3; % row number which has units specified
tbl = readtable(filename, opts); %NOTICE CHANGE HERE TOO
