The parameter "-1" is no longer supported, instead "infinity" should be used.

From SYSTEMD.RESOURCE-CONTROL(5)

TasksMax=N
Specify the maximum number of tasks that may be created in the unit. This ensures that the number of tasks accounted for the unit (see above) stays below a specific limit. This either takes an
absolute number of tasks or a percentage value that is taken relative to the configured maximum number of tasks on the system. If assigned the special value "infinity", no tasks limit is
applied. This controls the "pids.max" control group attribute. For details about this control group attribute, see Process Number Controller[7].

       The system default for this setting may be controlled with DefaultTasksMax= in systemd-system.conf(5).