without -r can't install requirements

with pip install -r requirements.txt i have this error
ERROR: Could not find a version that satisfies the requirement requirements.txt (from versions: none)
ERROR: No matching distribution found for requirements.txt

to solve this I added the -r option

-r, --requirement <file>    Install from the given requirements file. This option can be used multiple times.

Author: nagyzekkyandras

projects/Todo_app/Readme.md

@@ -7,6 +7,6 @@
 # To run app
 - Create virtual Environment
 - Install requirements
-`pip install requirements.txt`
+`pip install -r requirements.txt`
 - run app
 `py app.py`