Explanation
One of the possible reasons why an administrator is unable to add a new user in Salesforce is that the username is already in use by another user in any Salesforce org. Usernames must be globally unique across all Salesforce orgs, so the administrator needs to choose a different username for the new user. References:
https://help.salesforce.com/s/articleView?id=sf.users_add.htm&type=5

Explanation
A support process is a feature that allows administrators to define and enforce the stages that a case or work order must go through based on its record type. A support process determines which values are available for the status field for each record type. If a support process omits a certain value for the status field, such as Closed, then users will not be able to see or select that value when working with cases or work orders of that record type. References: https://help.salesforce.com/s/articleView?id=sf.customize_supporthome.htm&type=5

Explanation
Flow Builder is a tool that can be used to create an automated way to delete records based on field values.
Flow Builder can create flows that define the logic and actions for deleting records, such as finding records that match certain criteria and deleting them in bulk. Flows can be scheduled to run at regular intervals or triggered by other events or processes. References:
https://help.salesforce.com/s/articleView?id=sf.flow_builder.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.flow_concepts_delete.htm&type=5

Explanation
The Opportunity Stage Duration report is a standard report that shows how long opportunities spend in each stage before they are closed. It can be used to measure the average stage duration for all closed opportunities by grouping and summarizing the data by stage name and duration fields. Using process builder to capture the daily average on each opportunity is not feasible because it would require creating multiple fields and formulas on the opportunity object and updating them every day. Adding formula fields to track stages on each opportunity is also not practical because it would require creating multiple fields and formulas on the opportunity object and maintaining them every time a stage changes. Refreshing weekly reporting snapshots for closed opportunities is not necessary because the report can run on real-time data without snapshots.
References:
https://help.salesforce.com/s/articleView?id=sf.reports_opportunity_stage_duration_report.htm&type=5

Explanation
Forecasting is a feature that allows you to predict and plan the sales cycle from pipeline to closed sales, and manage sales expectations throughout your organization. Opportunity stages are the steps that an opportunity goes through as it moves from creation to close, and they determine the probability and forecast category of the opportunity. References:
https://help.salesforce.com/s/articleView?id=sf.forecasting3_overview.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.customize_opptystages.htm&type=5