user has enough access and source and subsite also and for respective list also but still getting below error while saving
Please confirm: What is the version of SharePoint you are using?
What are the user permissions on list level and site level? Do you see this error for other users in this list or other locations?
If this error happens only to this user (or random users), it is possibly caused by a permission mismatch on the user. For example, the user has access to a list with unique permissions but has limited or no access at the site level.
If that is the scenario, give the issued user READ permission at site level and then test again.
If this error happens to multiple or all users in your environment, check the physical memory (RAM) availability on your SharePoint front-end servers.
Restart SharePoint Search Host Controller service to quickly free up some memory space and then test again.
Please refer to this article for detailed information: The server was unable to save the form at this time SharePoint 2016.
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