If you've successfully created a 301 redirect, there's a few reasons it might not be working when you test it.
Fix the 'Redirect from' URL:
‘Redirect from’ URL must be a broken link
Broken URLs display an error message when visited, such as 'Page not found' or '404'. This means the requested page has been deleted, unpublished, moved or the URL has simply been misspelt.
Shopify does not support redirects of active pages. If the URL loads a valid webpage, then it contains active content and the redirect won't work because 301 redirects are intended to inform the browser that the webpage content has permanently moved from one location to another.
‘Redirect from’ URL must be a relative URL
A full URL (also called an absolute URL) includes the entire web address, starting with "https://".
A relative URL is the part of a full URL which points users to a specific page of the website. This includes the first forward slash after the domain name, and all the following text.
For example, if the full URL is https://example.com/products/shirt, the relative URL would be /products/shirt.
Shopify doesn't allow certain 'Redirect from' URLs
You can't redirect URLs that begin with the following prefixes: /apps, /application, /cart, /carts, /orders, or /services.
You can't redirect URLs that use fixed Shopify paths: /products, /collections, /collections/all.
'Redirect from' URL cannot be used in an existing redirect
If the 'Redirect from' URL is already being redirected, try editing or deleting the existing redirect on the URL Redirects page.
Fix the 'Redirect to' URL:
‘Redirect to’ URL must be the correct type of URL
To redirect visitors to a different page on your primary website domain use a relative URL e.g. /products/shirt
To redirect visitors to a subdomain, secondary domain or a different website use a full URL e.g. https://your-shopify-store.com/products/shirt
‘Redirect to’ URL must load a valid website
If the 'Redirect to' URL is a broken link, the redirect will lead to a 404 error page.
'Redirect to' URL cannot be a broken link that is already being redirected
Shopify does not allow 'chaining' of redirects. This means broken link 'A' cannot be redirected to broken link 'B' if 'B' is already being redirected to an active page, 'C'.
In this context, try redirecting broken link 'A' to active page 'C'.