Looking better at the messages in the log files, I have seen an error like that:
memcached.log.000001.txt:2021-09-09T18:37:33.063209+00:00 CRITICAL Failed to initialise - CPU with SSE4.2 extensions is required - terminating.
This error is related to the missing sse4_2 cpu flags . Since I installed the couchbase server on a virtual machine , the issue has been solved out by changing the VM processor type, to one that could use the sse4_2 flags , instead of the generic one previously set. So , once the VM has been restarted, a cpuinfo reported the cpu missed flag:
cat /proc/cpuinfo
...omissis...
model name : Intel Xeon Processor (Skylake)
...omissis..
flags : fpu .................... pclmulqdq ssse3 fma cx16 pcid sse4_1 sse4_2 x2apic movbe popcnt .............
...omissis...
After that , I managed to start the cluster and join other nodes to it