The Series.append function has been removed since pandas=2.0 (see Issue #35407). As of today, pandas_ta version 0.3.14b0 works with pandas=1.5.3 but produces this bug if you use Pandas 2+. Apparently, this has been fixed in the development branch (see Issue #695). Try downgrading Pandas or switching to the development branch of Pandas TA to resolve this issue.