The reason for this error is:
the version of NFS used in uboot is V2, while the version of NFS used in Ubuntu is V3, V4 and above. As a result, uboot can no longer find files in the NFS server.

The solution is to make NFS in buntu compatible with v2. Modify the /etc/default/NFS kernel server file.

Open the file: sudo VIM/etc/default/NFS kernel server (note that sudo or root permissions are required), and modify it as follows:

# Number of servers to start up
RPCNFSDCOUNT=" -V 2 8"

# Runtime priority of server (see nice(1))
RPCNFSDPRIORITY=0

# Options for rpc.mountd.
# If you have a port-based firewall, you might want to set up
# a fixed port here using the --port option. For more information, 
# see rpc.mountd(8) or http://wiki.debian.org/SecuringNFS
# To disable NFSv4 on the server, specify '--no-nfs-version 4' here
RPCMOUNTDOPTS="-V 2 --manage-gids"

# Do you want to start the svcgssd daemon?It is only required for Kerberos
# exports. Valid alternatives are "yes" and "no"; the default is "no".
NEED_SVCGSSD=""

# Options for rpc.svcgssd.
RPCSVCGSSDOPTS="--nfs-version 2,3,4 --debug --syslog"

Restart the NFS server. sudo service nfs-kernel-server restart