Hi, and welcome!
Please see the FAQ: What's a reproducible example (`reprex`) and how do I do one? Using a reprex, complete with representative data will attract quicker and more answers. Because this question is more conceptual, however, it's not needed
First, it's poor etiquette to contact the authors usually, unless it is a likely bug in the program.
The way to work this problem is by looking at help(pvalueMC) to see what its arguments are asking for.
null.law.index .... index of the distribution to be tested (the null hypothesis distribution), as given by function getindex.
In the function signature we see that there is no default value (`null.law.index = SOMETHING), so we have to provide one.
What is it? It's a code for the statistical null hypothesis.
Where can the code lookups be found?
In getindex.
What does getindex say?
library(PoweR)
#> Loading required package: parallel
#> Loading required package: Rcpp
# getindex(law.indices = NULL, stat.indices = NULL)
getindex()
#> Index Law Nbparams Default1 Default2 Default3
#> 1 1 Laplace(mu,b) 2 0.000000 1 NA
#> 2 2 Normal(mu,sigma) 2 0.000000 1 NA
#> 3 3 Cauchy(mu,sigma) 2 0.000000 1 NA
#> 4 4 Logistic(mu,sigma) 2 0.000000 1 NA
#> 5 5 Gamma(shape,rate) 2 2.000000 1 NA
#> 6 6 Beta(a,b) 2 1.000000 1 NA
#> 7 7 Uniform(a,b) 2 0.000000 1 NA
#> 8 8 Student-t(df) 1 1.000000 NA NA
#> 9 9 Chi-squared(df) 1 1.000000 NA NA
#> 10 10 Lognormal(logmean,logsd) 2 0.000000 1 NA
#> 11 11 Weibull(shape,scale) 2 1.000000 1 NA
#> 12 12 ShiftedExp(l,rate) 2 0.000000 1 NA
#> 13 13 U^{j+1} 1 1.000000 NA NA
#> 14 14 AveUnif(k,a,b) 3 2.000000 0 1.0
#> 15 15 UUnif(j) 1 1.000000 NA NA
#> 16 16 VUnif(j) 1 1.000000 NA NA
#> 17 17 JSU(mu,sigma,nu,tau) 4 0.000000 1 0.0
#> 18 18 Tukey(l) 1 1.000000 NA NA
#> 19 19 LoConN(p,m) 2 0.200000 3 NA
#> 20 20 JSB(g,d) 2 0.000000 1 NA
#> 21 21 SkewN(xi,omega,alpha) 3 0.000000 1 0.0
#> 22 22 ScConN(p,d) 2 0.200000 2 NA
#> 23 23 GP(mu,sigma,xi) 3 0.000000 1 0.0
#> 24 24 GED(mu,sigma,p) 3 0.000000 1 1.0
#> 25 25 Stable(alpha,beta,c,mu) 4 1.000000 0 1.0
#> 26 26 Gumbel(mu,sigma) 2 1.000000 1 NA
#> 27 27 Frechet(mu,sigma,alpha) 3 0.000000 1 1.0
#> 28 28 GEV(mu,sigma,xi) 3 0.000000 1 0.0
#> 29 29 GArcSine(alpha) 1 0.500000 NA NA
#> 30 30 FoldN(mu,sigma) 2 0.000000 1 NA
#> 31 31 MixN(p,m,d) 3 0.500000 0 1.0
#> 32 32 TruncN(a,b) 2 0.000000 1 NA
#> 33 33 Nout(a) 1 1.000000 NA NA
#> 34 34 GEP(t1,t2,t3,crit) 4 0.500000 0 1.0
#> 35 35 Exponential(lambda) 1 1.000000 NA NA
#> 36 36 ALaplace(mu,b,k) 3 0.000000 1 2.0
#> 37 37 NIG(alpha,beta,delta,mu) 4 1.000000 0 1.0
#> 38 38 APD(theta,phi,alpha,lambda) 4 0.000000 1 0.5
#> 39 39 modAPD(mu,sigma,theta1,theta2) 4 0.000000 1 0.5
#> 40 40 LPtn(alpha,mu,sigma) 3 1.959964 0 1.0
#> Default4
#> 1 NA
#> 2 NA
#> 3 NA
#> 4 NA
#> 5 NA
#> 6 NA
#> 7 NA
#> 8 NA
#> 9 NA
#> 10 NA
#> 11 NA
#> 12 NA
#> 13 NA
#> 14 NA
#> 15 NA
#> 16 NA
#> 17 5e-01
#> 18 NA
#> 19 NA
#> 20 NA
#> 21 NA
#> 22 NA
#> 23 NA
#> 24 NA
#> 25 0e+00
#> 26 NA
#> 27 NA
#> 28 NA
#> 29 NA
#> 30 NA
#> 31 NA
#> 32 NA
#> 33 NA
#> 34 1e-06
#> 35 NA
#> 36 NA
#> 37 0e+00
#> 38 2e+00
#> 39 2e+00
#> 40 NA
#> Index Stat Alter Nbparams
#> 1 1 K-S 3 0
#> 2 2 AD^* 3 0
#> 3 3 Z_C 3 0
#> 4 4 Z_A 3 0
#> 5 5 P_S 3 0
#> 6 6 K^2 3 0
#> 7 7 JB 3 0
#> 8 8 DH 3 0
#> 9 9 RJB 3 0
#> 10 10 T_{Lmom} 3 0
#> 11 11 T_{Lmom}^{(1)} 3 0
#> 12 12 T_{Lmom}^{(2)} 3 0
#> 13 13 T_{Lmom}^{(3)} 3 0
#> 14 14 BM_{3-4} 3 0
#> 15 15 BM_{3-6} 3 0
#> 16 16 T_{MC-LR} 3 0
#> 17 17 T_w 0,1,2 0
#> 18 18 T_{MC-LR}-T_w 3 0
#> 19 19 T_{S,5} 3 0
#> 20 20 T_{K,5} 3 0
#> 21 21 W 4 0
#> 22 22 W' 4 0
#> 23 23 tilde{W} 4 0
#> 24 24 D 0,1,2 0
#> 25 25 r 4 0
#> 26 26 CS 3 0
#> 27 27 Q 0,1,2 0
#> 28 28 Q-Q* 0,1,2 0
#> 29 29 BCMR 3 0
#> 30 30 beta_3^2 3 0
#> 31 31 T^*(alpha) 1 1
#> 32 32 I_n 3 0
#> 33 33 R_{sJ} 3 0
#> 34 34 Q* 0,1,2 0
#> 35 35 R_n 3 0
#> 36 36 X_{APD} 3 0
#> 37 37 Z_{EPD} 0,1,2 0
#> 38 38 GLB 3 0
#> 39 39 V_3-ML 0,1,2 0
#> 40 40 V_4-ML 0,1,2 0
#> 41 41 S 3 0
#> 42 42 A^2 3 0
#> 43 43 W^2 3 0
#> 44 44 U^2 3 0
#> 45 45 sqrt{n}D 3 0
#> 46 46 V 3 0
#> 47 47 T_{n,a}^{(1)}-MO 3 1
#> 48 48 T_{n,a}^{(1)}-ML 3 1
#> 49 49 T_{n,a}^{(2)}-MO 3 1
#> 50 50 T_{n,a}^{(2)}-ML 3 1
#> 51 51 T_{m,n}^{V} 4 1
#> 52 52 T_{m,n}^{E} 4 1
#> 53 53 T_{m,n}^{C} 4 1
#> 54 54 hat{G}_n 3 0
#> 55 55 V_3 0,1,2 0
#> 56 56 V_4 0,1,2 0
#> 57 57 K_1 3 0
#> 58 58 T 3 0
#> 59 59 Z 3 0
#> 60 60 K 3 0
#> 61 61 DLLap1 0,1,2 0
#> 62 62 DLLap2 0,1,2 0
#> 63 63 D_n 3 0
#> 64 64 W_{n}^{2} 3 0
#> 65 65 A_{n}^{2} 3 0
#> 66 66 C_n 3 0
#> 67 67 K_n 3 0
#> 68 68 T_1 3 0
#> 69 69 T_2 3 0
#> 70 70 G(n) 3 0
#> 71 71 Q 3 0
#> 72 72 2nI^{lambda} 3 1
#> 73 73 M(n) 3 0
#> 74 74 L_{n}^{(m)} 4 1
#> 75 75 S_{n}^{(m)} 3 1
#> 76 76 H(m,n) 4 1
#> 77 77 A^{*}(n) 3 0
#> 78 78 D_{n,m}(phi_lambda) 3 2
#> 79 79 E_{m,n} 3 1
#> 80 80 T_{n,m}^{lambda} 3 2
#> 81 81 Z_A 3 0
#> 82 82 Z_C 3 0
#> 83 83 t test 0,1,2 1
#> 84 85 DLLap3 3 0
#> 85 86 T(alpha_1,alpha_2) 3 1
#> 86 87 T_n 3 0
#> 87 88 T^{LS}(alpha) 3 1
#> 88 89 T^{LS}_3(mu,alpha) 3 2
#> 89 90 T^{LS}_4(alpha,mu) 3 2
#> 90 91 GV_1 0,1,2 0
#> 91 92 GV_2 0,1,2 0
#> 92 93 Ho_K 0,1,2 0
#> 93 94 Ho_U 0,1,2 0
#> 94 95 Ho_V 0,1,2 0
#> 95 96 Ho_W 0,1,2 0
#> 96 97 SR^* 0,1,2 0
Created on 2020-04-07 by the reprex package (v0.3.0)
That's a lot of options, and which one to use depends on the design of the analysis.