The Tdist function doesn’t work with negative T scores. This is an oversight in EXCEL it’s not your fault.
The workaround, however, is simple.
Use the abs function in entering the T-observed into the Tdist function.
The Function, abs(X) fixes it so that even if X is negative, a positive number is obtained.
Use this to go around the bug in the Tdist function that keeps it from giving a result when the Tobserved is negative.
Note that “Absoluting” a cell reference using the function key, F4, is a totally different matter. F4 puts $ signs on a cell reference so that it is absolute instead of relative, and won't change when you copy the formula to a different cell.