yaml - ScannerError: mapping values are not allowed here

At the root of your YAML document you have a mapping with
key test_name and as value the start of a plain scalar
tavern .....

The
parser expects a key, with the same indent as the first line, on the second line or a
continuation of your plain scalar from the first line. The second line is empty, so it
continues with the same expectations on the third line. There it finds an
- which is further indented than the beginning of
test_name, so it is not a key, but part of the plain scalar.
Then it finds name also part of a the plain scalar started on
the first line and then : (colon +
space).

But that colon+space is not allowed in
plain scalar, as that gives potential ambiguity with the start of another key-value
pair.

The unlikely solution is that
you put double quotes before tavern and at the end of the YAML
document.

More likely you should include a key
for which the structure starting with - name: list this is the
value. E.g.:

test_name: tavern
            poc

stages:
 - name: list

            request:

(as
from the second entry you get from googling "tavern yaml")