Please follow below tips to troubleshoot the issue.
1.Make sure the account you use must have administrator editing privileges to associate hubs to each other.
2.Clear the browser cache, then associate with a hub site again. If the issue persists, try to associate with a hub site in another browser or PC.
3.Try to associate with other hub sites, check whether the issue occurs with all hub sites.
4.Use PnP PowerShell to associate with a hub site.
#Config Variables
$TenantSiteUrl = "https://tenant-admin.sharepoint.com"
$HubSiteURL = "https://tenant.sharepoint.com/sites/test"
$SiteURL = "https://tenant.sharepoint.com/sites/test1"
#Connect to PnP Online
Connect-PnPOnline -Url $TenantSiteUrl -Credentials (Get-Credential)
#Associate Site collection with HubSite
Add-PnPHubSiteAssociation -Site $SiteURL -HubSite $HubSiteURL
5.Go to the SharePoint admin center -> Active sites -> Choose the site -> Hub -> Associate with a hub site.
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