That is not how you access data inside Tables. Go through this for more information - Access Data in Tables
%% Provide the filename with extension
P=readtable('data1.xlsx');
%% Use dot indexing to access the data
X=log10(P.Var1);
Y=log10(P.Var2); % convert both variables to log's
b=polyfit(X,Y,1); % estimate coefficients
yhat=10.^[polyval(b,[X(1) X(end)])]; % evaluate at end points
%% Variable names are capital
figure
loglog(X,Y,'r')
hold on
plot([X(1) X(end)],yhat,'b') % add fitted line
