Convert string to formula

Question

Hi,

The synopsis of my problem is I am generating a string by combining some constant strings and numbers. I am passing this string into the sum function so that it can calculate the summation. When I am using a string by itself in a cell ( with a = sign at the front of course) and hitting enter retrieves the value. But when I am passing the string not occurring. So I tried to use Evaluate function as a named function, VBA, directly in the cell. None of them works. I am using Office 365 ( Excel 2016). Any help on this?

The string is ="'"&AC15&"'!$F$"&AC18&":OFFSET("&"'"&AC15&"'!$F$"&AC18&",0,0)"

And the output of the string is 'G:CrushDaily plant reports2019[05-2019.xls]C vol'!$F$35:OFFSET('G:CrushDaily plant reports2019[05-2019.xls]C vol'!$F$35,0,0)

I stored the above string in AC19 cell

I used sum function like this =sum(AC19)

Which returns zero, but actually at 'G:CrushDaily plant reports2019[05-2019.xls]C vol'!$F$35 the value is 2630. Any mistake in the syntax?