Free memory will always be available unless nothing is done to prevent memory shortage. Sap hana memory would have been sized initially according to data requirements and data growth estimation.
There should be plenty of free space in a sap hana healthy system.
However, memory can become suddenly short in the following situations:
A common and regular problem is a peak memory usage which takes most of the free memory in order to run a specific job requirement.
It is happening in most cases because queries were not optimized and validations were not performed.
Keep in mind, when too many bad queries are executed at the same time, the overall tuning may become impossible. Tuning single query is therefore very important.
Memory shortage could also be shown in an healthy system, where data is being added month after month, year after year and one day memory alerts start to show up.
That situation can be as difficult to sort out. If hardware can accommodate more physical memory, there is no real issues. Just getting a new sap license and new physical memory.
Otherwise, it become a database migration project from one host to another bigger one. That will not be possible in a short time scale.