= before the IF() is the first problem. The
= is only needed at the beginning of the formula, not when nesting functions.
Remove the Concat() from the formula. It does not look like you want to concatenate anything, since the IF() is the only argument and your desired outcome is just a number.
The last nested IF does not have a "false" parameter. If F26 contains any other text, the formula will return "FALSE", so you may want to control that, too.
All suggestions rolled into this:
=IF(F26 = " Kb","0",IF(F26 = "16 Kb","16",IF(F26 = "64 Kb","64","other")))