Packages implementing post-fit procedures define helper functions which may not handle spaMM's fit objects, or which have not always handled them, or which can handle them correctly only with some non-default arguments. This documentation topic gives further directions to apply some such post-fit procedures (from packages DHARMa, RLRsim, multcomp and lmerTest) to these fit objects.
For other procedures not considered here, diagnosing a failure in a debugging session may suggest a simple solution (as it did for multcomp::glht).
Details
For multiple comparison procedures by multcomp::glht, one has to explicitly give the argument coef.=fixef.HLfit (see Examples; fixef.HLfit is the spaMM method for the generic function fixef);
For DHARMa plots, see Details of plot.HLfit;
For using RLRsim::RLRTSim(), see get_RLRTSim_args.
For using lmerTest::contest() or lmerTest::anova(), see as_LMLT.