sp_Blitz: Poison wait detected and too much free memory
This is not an over-provisioning issue or even a memory misconfiguration. 

RESOURCE_SEMAPHORE: "Occurs when a query memory request cannot be granted immediately due to other concurrent queries. High waits and wait times may indicate excessive number of concurrent queries, or excessive memory request amounts."

Depending on which version and build you are using of SQL Server, you may be able to query for large memory grants so that you know where to start on which queries are causing a problem. If you are on SQL 2012 SP3 or greater, SQL 2014 SP2 or greater or SQL 2016 RTM or greater, then you can query for this information. See the query at the bottom of New memory grant query hint MIN_GRANT_PERCENT came to rescue.

Check out Query Plans: Memory Grants and High Row Estimates for more information on memory grants, which is the link inside the link sp_Blitz directed you to.