'*myanchor' is a YAML scalar and will be loaded as string. If you want to refer to the node with the anchor &myanchor, you must not use quotes.
If you drop the quotes, the structure in My Job will be
My Job:
script:
- 'echo "Some Text"'
- - 'echo "Some Text"'
It is likely that nested sequences are not supported for the content of script: (this is not a limitation of YAML per se, but a limitation of the tool that processes this structure).
To fix it, you need to put the anchor on the sequence item instead of the list:
.myanchor:
- &myanchor 'echo "Some Test"'
My Job:
script:
- 'echo "Some Text"'
- *myanchor
Now the problem is probably that you want to reference multiple items with *myanchor and put each of them as item directly into the script: sequence. That is simply not possible with YAML so you need to find another solution – perhaps preprocessing your YAML with a templating engine (lots of YAML's heavy users like e.g. Ansible and SaltStack do this).