If the plugins are not in the same directory included in the PATH variable, then you can run into this problem. Either set your path to include the correct directory or set the AFNI_PLUGINPATH variable. This information is extracted from the README.environment file.

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Variable: AFNI_PLUGINPATH
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This variable should be the directory in which AFNI should search
for plugins. If there is more than one appropriate directory, they
can be separated by colons, as in

setenv AFNI_PLUGINPATH /directory/one:/directory/two

If this variable is not set, then AFNI will use the PATH variable
instead. This will waste time, since most directories in the PATH
will not have plugins. On some systems, using the PATH has been
known to cause problems when AFNI starts. I believe this is due to
bugs in the system library routines (e.g., dlopen) used to manage
dynamically loaded shared objects.

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Variable: AFNI_NOPLUGINS
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If this YES/NO variable is set to YES, then AFNI will not try to
read plugins when it starts up. The command line switch "-noplugins"
will have the same effect.