Simply waiting often gives the system time to free resources. However, if this message occurs often on a system, reconfigure the kernel and allow more processes. To increase the size of the process table, increase the value of MAXUSERS in the /etc/system file. The default MAXUSERS value is the amount of main memory in Mbytes, minus 2.
If one user is not allowed to create any more processes, that user has probably exceeded the memory size limit; see the limit(1) man page for details.