You don't pass the table to regress but the variables to be used in the regression -- then you won't run into the issue inside regress.
And you DEFINITELY DO NOT WANT TO BE MUCKING INSIDE THE SUPPLIED REGRESS FUNCTION!!!!
We don't know the function you're trying to fit nor the variable names in your table, but assuming
for variables X and Y in the table and a linear model plus intercept, then the syntax for regress would be
where the table variable is t. Use your table variable name and variable names within the table, of course.
If you have the Curve Fitting Toolbox besides Statistics, I would suggest that the fit function in it is a little more user friendly than the core regress function. Lacking it, see the Alternative Functionality section of the documentation for regress that suggests using LinearModel instead for similar reasons/purposes.
Read the section in the documentation for table on how to address data within a table for the details of using tables and which forms of addressing return the variables as native type, tables, etc., ... But, in particular note that addressing a table variable with parentheses returns another table of the addressed rows and columns within the table which is probably the root cause of your troubles.