This is not true. You can pray tahajjud after taraweeh. Please read
this for a detailed explanation:

Kindly refer to the following link(s):

https://islamqa.info/en/65702

As for salatul tasbeeh, please watch this:

Kindly refer to the following link(s):

https://www.youtube.com/watch?v=sO5gyDDT5fk

can we pray tahajud or salatul tasbeeh after praying 12 rakas of taraveeh? I was told that it is like exceeding sunnah and not permissible and taraveeh is equivalent to tahajud. Kindly clear the concept with reference if possible