The variable (column) is a cell array of strings, so you need to use strcmpi() or strcmp() instead of ==. See this demo:
% Create Table from Workspace Variables
% Define workspace variables with the same number of rows.
LastNames = {'Johnson';'Roberson';'Simon';'Greene';'Analyst'};
Age = [38;43;38;40;49];
Height = [71;69;64;67;64];
Weight = [176;163;131;133;119];
BloodPressure = [124 93; 109 77; 125 83; 117 75; 122 80];
% Create a table, |T|, as a container for the workspace variables.
T = table(LastNames, Age,Height,Weight,BloodPressure)
% Extract the LastName column so we can see it's a cell array.
ca = T.LastNames
whos ca
% See it work with strcmpi()
rowsToExtract = strcmpi(ca, 'Johnson')
output = T(rowsToExtract, :)
% Now see it fail with ==
matches = (ca == 'Johnson')
