Frankly it's mostly likely a misleading message in the console. The new console sometimes shows 'Pending' as 'couldn't fetch' incorrectly.
... its normal for sitemaps to be 'Pending'. It can be hours, it can be weeks, in Pending. It varies by sitemap.
I.e. NOT that it tried and failed to fetch. It simply hasn't tried YET. It's 'hasn't fetched' rather than 'couldn't'
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But first need to check if there REALLY is some sort of Fetching issue. ... the 'Live Test' function inside URL Inspection can be used for this. Because it could also have tried and failed.
Try Inspecting the sitemap URL itself, then click the 'Live Test' button. See what it reports under 'Page Fetch' (in 'Crawl' section) - ignore the messages about Indexing!
(It doesnt matter that if it says not indexed or known on the first screen - it's a sitemap not a page, so shouldn't be indexed!, You just using URL Inspection to use the 'Live Test' function. ... Also as it's a sitemap, NOT a page, DON'T use the 'Request Indexing' button!)
If Google can fetch it, it's most likely just Pending, which case just wait.