.difference_sign_test: no visible global function definition for
  ‘pnorm’
.forecast.diff: no visible global function definition for ‘diffinv’
.ljung_box_test: no visible global function definition for ‘pchisq’
.mcleod_li_test: no visible global function definition for ‘pchisq’
.plot.forecast: no visible global function definition for ‘is.ts’
.plot.forecast: no visible global function definition for ‘time’
.plot.forecast: no visible global function definition for ‘frequency’
.plot.forecast: no visible global function definition for ‘start’
.plot.forecast: no visible global function definition for
  ‘plot.default’
.plot.forecast: no visible global function definition for ‘lines’
.rank_test: no visible global function definition for ‘pnorm’
.test.aacvf: no visible global function definition for ‘ARMAacf’
.test.acvf: no visible global function definition for ‘acf’
.turning_point_test: no visible global function definition for ‘pnorm’
arma: no visible global function definition for ‘arima’
arma: no visible global function definition for ‘coef’
arma: no visible global function definition for ‘vcov’
burg: no visible global function definition for ‘ar.burg’
hannan: no visible global function definition for ‘ar.yw’
periodogram: no visible global function definition for ‘plot’
periodogram: no visible global function definition for ‘par’
plota : plot.data.acf: no visible global function definition for ‘par’
plota : plot.data.acf: no visible global function definition for ‘plot’
plota : plot.data.acf: no visible global function definition for
  ‘abline’
plota : plot.data.acf: no visible global function definition for
  ‘legend’
plota : plot.model.acf: no visible global function definition for ‘par’
plota : plot.model.acf: no visible global function definition for
  ‘ARMAacf’
plota : plot.model.acf: no visible global function definition for
  ‘plot’
plota : plot.model.acf: no visible global function definition for
  ‘abline’
plota : plot.model.acf: no visible global function definition for
  ‘legend’
plota : plot.combo.acf: no visible global function definition for ‘par’
plota : plot.combo.acf: no visible global function definition for
  ‘plot’
plota : plot.combo.acf: no visible global function definition for
  ‘ARMAacf’
plota : plot.combo.acf: no visible global function definition for
  ‘lines’
plota : plot.combo.acf: no visible global function definition for
  ‘abline’
plota : plot.combo.acf: no visible global function definition for
  ‘legend’
plotc: no visible global function definition for ‘is.ts’
plotc: no visible global function definition for ‘time’
plotc: no visible global function definition for ‘plot.default’
plotc: no visible global function definition for ‘frequency’
plotc: no visible global function definition for ‘start’
plotc: no visible global function definition for ‘lines’
plots : plot.data.spectrum: no visible global function definition for
  ‘fft’
plots : plot.data.spectrum: no visible global function definition for
  ‘plot’
plots : plot.model.spectrum: no visible global function definition for
  ‘plot’
sim: no visible global function definition for ‘rnorm’
sim: no visible global function definition for ‘filter’
smooth.rank: no visible global function definition for ‘fft’
test: no visible global function definition for ‘par’
test: no visible global function definition for ‘acf’
test: no visible global function definition for ‘plot’
test: no visible global function definition for ‘abline’
test: no visible global function definition for ‘qqnorm’
yw: no visible global function definition for ‘ar.yw’
Undefined global functions or variables:
  ARMAacf abline acf ar.burg ar.yw arima coef diffinv fft filter
  frequency is.ts legend lines par pchisq plot plot.default pnorm
  qqnorm rnorm start time vcov
Consider adding
  importFrom("graphics", "abline", "legend", "lines", "par", "plot",
             "plot.default")
  importFrom("stats", "ARMAacf", "acf", "ar.burg", "ar.yw", "arima",
             "coef", "diffinv", "fft", "filter", "frequency", "is.ts",
             "pchisq", "pnorm", "qqnorm", "rnorm", "start", "time",
             "vcov")
to your NAMESPACE file.