I can confirm that this appears to be a bug.
Fresh installation of Mojave in a VM with Xcode, Xcode command line tools, Lazarus 2.0.6 and FPC 3.0.4.
Attachment 1 - shows Printer4Lazarus is installed
Attachment 2 - shows Printer4Lazarus as "online" and not installed
Attachment 3 - shows attempt to Install from Project Inspector
Error message when compiling otherwise blank project after adding printer4lazarus to uses clause of project source:
Compile Project, Target: project1: Exit code 1, Errors: 1
project1.lpr(10,10) Fatal: Cannot find printer4lazarus used by project1. Check if package Printer4Lazarus is in the dependencies of the Project Inspector.
Directory listing of /Library/Lazarus/components/printers/lib/x86_64-darwin/cocoa showing printer4lazarus:
-rw-r--r-- 1 root wheel 546 6 Nov 2019 Printer4Lazarus.compiled
-rw-r--r-- 1 root wheel 265188 5 Nov 2019 cocoaprndelegate.o
-rw-r--r-- 1 root wheel 9920 5 Nov 2019 cocoaprndelegate.ppu
-rw-r--r-- 1 root wheel 201220 5 Nov 2019 osprinters.o
-rw-r--r-- 1 root wheel 30780 5 Nov 2019 osprinters.ppu
-rw-r--r-- 1 root wheel 1776 6 Nov 2019 printer4lazarus.o
-rw-r--r-- 1 root wheel 1456 6 Nov 2019 printer4lazarus.ppu
-rw-r--r-- 1 root wheel 11836 5 Nov 2019 printer4lazstrconst.o
-rw-r--r-- 1 root wheel 8182 5 Nov 2019 printer4lazstrconst.ppu
-rw-r--r-- 1 root wheel 4345 5 Nov 2019 printer4lazstrconst.rsj
-rw-r--r-- 1 root wheel 270012 5 Nov 2019 printersdlgs.o
-rw-r--r-- 1 root wheel 12179 5 Nov 2019 printersdlgs.ppu
-rw-r--r-- 1 root wheel 9505 5 Nov 2019 printersdlgs.res
As the original poster indicated, I cannot add Printer4Lazarus using the Project Inspector.
The workaround was to manually add it to the project.lpi file changing:
<RequiredPackages Count="1">
<Item1>
<PackageName Value="LCL"/>
</Item1>
</RequiredPackages>
to:
<RequiredPackages Count="2">
<Item1>
<PackageName Value="Printer4Lazarus"/>
</Item1>
<Item2>
<PackageName Value="LCL"/>
</Item2>
</RequiredPackages>
I also tried Lazarus 2.1.0 (trunk r63272) with FPC v3.3.1 (trunk r45525) on Mojave with the same results.