Issue 301 Re directs

[nickthepen]

Server operating system version

CloudLinux 7.9

Plesk version and microupdate number

18.0.62

I have just built a new website using Shopify changing from my old site on Sellerdeck

I am trying to redirects from old to new site in .htaccess but am getting very inconsistent results.
eg:
Redirect 301 /https://www.oldsite.co.uk/acatalog/waterman-pen.html https://www.newsite.shop/collections/waterman-pen This re direct works

Redirect 301 /https://www.oldsite.co.uk/acatalog/faber-castell-grip-marker-pen.html https://www.newsite.shop/collections/faber-castell-pens This re direct just stays on the original page

Redirect 301 /https://www.oldsite.co.uk/acatalog/parker-pen-refills.html https://www.newsite.shop/collections/parker-pen-refills This re direct directs to /acatalog/parker-pen-refills Which is a page that has never been created

Redirect 301 /https://www.oldsite.co.uk/acatalog/pen-again-twist-write-pencil-2-pack-p00073.html https://www.newsite.shop/products/pen-again-twist-write-pencil-2-pack This just comes up with a 500 Internal Server Error

I'm not an expert and thought this would be fairly straight forward as I have successfully done redirects before. The file size is currently 9.6KB

Thanks
Nick