Unable to perform assignment because the indices on the left side are not compatible with the size of the right side.

ew = [-1 0 0 3]; % [<begin pre> <end pre> <begin post> <end post>]
for abc = 1:size(ew,2)
    [~, ix(abc)] = min(abs(tw-ew(abc)));
end

clearvars window table_rat table_sex table_trial table_licklat table_numlicks table_pks table_locs table_drug
row = 1;
for abc = animalnumber
    for xyz = 1:data(abc).ntrain
        wind = data(abc).trainOn(xyz):data(abc).trainOn(xyz)+ix(4);
        if isempty(find(data(abc).locs>wind(1) & data(abc).locs<wind(end)))
        else
            table_rat(row,1) = {data(abc).animal};
            table_sex(row,1) = {data(abc).sex};
            table_drug(row,1) = str2num(data(abc).conditions(1));
            table_trial(row,1) = xyz;
            table_licklat(row,1) = data(abc).licklat(xyz);
            table_numlicks(row,1) = data(abc).numolicks(xyz);
            table_pks(row,1) = max(data(abc).pks(data(abc).locs>wind(1) & data(abc).locs<wind(end)));
            table_locs(row,1) = data(abc).locs(data(abc).pks==table_pks(row,1));
            table_pkfreq(row,1) = numel(data(abc).pks(data(abc).locs>wind(1) & data(abc).locs<wind(end)))/(size(wind,2)/fsg1);
            row = row + 1;
        end
    end
end
vars = {'rat','sex','drug','trial','licklat','numlicks','locs','pkmag','pkfreq'};
table_pkmagtrial = table(table_rat,...
    table_sex,...
    table_drug,...
    table_trial,...
    table_licklat,...
    table_numlicks,...
    table_locs,...
    table_pks,...
    table_pkfreq,...
    'VariableNames',vars);

writetable(table_pkmagtrial,'pkmagtrial3s.csv','Delimiter',',')
clearvars table_rat table_drug table_trial table_locs table_pks

[Jan]

You forgot to ask a question and to post a copy of the complete error msssage. Please format your code properly in addition.

Do not waste the time with clearvars, because it is rarely useful.