It happened that my first blog post ever will be on an issue I hit with my nested lab at home. I was trying to install NSX-T 3.1, when I noticed the installation failed on one out of 3 hosts:

Upon further investigation I have found the following errors in the /var/log/esxiupdate.log on the host itself:

2020-12-08T18:01:47Z esxupdate: 68993: root: ERROR: vmware.esximage.Errors.InstallationError: Error in running ['/etc/init.d/nsx-datapath-rp', 'start', 'install']:
2020-12-08T18:01:47Z esxupdate: 68993: root: ERROR: Return code: 1
2020-12-08T18:01:47Z esxupdate: 68993: root: ERROR: Output: nsx-datapath-rp start
2020-12-08T18:01:47Z esxupdate: 68993: root: ERROR: rp added for nsx-datapath
2020-12-08T18:01:47Z esxupdate: 68993: root: ERROR: FAILED to setmemconfig for nsx-datapath to 300MB. Retrying ..
2020-12-08T18:01:47Z esxupdate: 68993: root: ERROR: FAILED to setmemconfig for nsx-datapath to 300MB. Retrying ..
2020-12-08T18:01:47Z esxupdate: 68993: root: ERROR: FAILED to setmemconfig for nsx-datapath to 300MB. Retrying ..
2020-12-08T18:01:47Z esxupdate: 68993: root: ERROR: FAILED to setmemconfig for nsx-datapath to 300MB. Retrying ..
2020-12-08T18:01:47Z esxupdate: 68993: root: ERROR: FAILED to setmemconfig for nsx-datapath to 300MB. Retrying ..
2020-12-08T18:01:47Z esxupdate: 68993: root: ERROR: FAILED to setmemconfig for nsx-datapath
2020-12-08T18:01:47Z esxupdate: 68993: root: ERROR: Resource pool 'host/vim/vmvisor/nsx-datapath' released.
2020-12-08T18:01:47Z esxupdate: 68993: root: ERROR:
2020-12-08T18:01:47Z esxupdate: 68993: root: ERROR: It is not safe to continue. Please reboot the host immediately to discard the unfinished update.

From the above error it is clear the host failed to set the memory reservation for nsx-datapath to 300MB, hence the installation failed.

I have checked the VM settings of all 3 hosts and noticed esxi3 is running on 4GB of ram, while esxi1 and esxi2 are configured with 16GB. Quick check of the official documentation confirmed the required Memory for configuring an ESXi as a Transport Node is 16GB:

After increasing the memory to 16GB and clicking on the Resolve button the NSX-T 3.1 got successfully installed.